LT7003B - Semiconductor optoelectronic devices characterization apparatus - Google Patents

Abstract

The invention belongs to the field of measuring the spectral characteristics of semiconductor optoelectronic devices and can be used to study the performance characteristics of various optoelectronic components. The proposed apparatus includes a femtosecond laser, a mirror that divides the laser beam into two parts, where the higher intensity beam part is directed to the test device, and the lower intensity beam part activates the detector into which the radiation generated and reflected by the test device enters. In order to extend the measuring wavelength range and increase the accuracy of the spectral measurements, the apparatus has an optical parametric amplifier through which the part of the beam directed to the test device passes and in which a beam of variable wavelength femtosecond light pulses is generated. Once in the test device, this beam generates pulses of terahertz (THz) radiation, which, together with the pulses reflected from the device, enter a detector that registers the pulses of THz radiation. The apparatus determines the spectral characteristics of the device under investigation according to the shape of the THz pulses recorded in the detector over time.

Description

TECHNICAL FIELD

This invention belongs to the field of measuring the spectral characteristics of semiconductor optoelectronic devices and can be used in the measurement of performance characteristics of various optoelectronic components, including multi-junction solar cells.

STATE OF THE ART

Usually, the characteristics of semiconductor optoelectronic devices are measured by illuminating the device with a light beam of various wavelengths and measuring the electrical signal generated at the contacts of the device. However, for optoelectronic devices such as multi-junction solar cells (DSSEs) monolithically integrated in a single device, similar measurements using electrical contacts are impossible because the inner junction layers of the device are inaccessible. Therefore, this problem is solved by using several light sources of different wavelengths.

One such DSSE characterization device is described in the paper (S.W. Lim et al, "Analysis of spectral photocurrent response from multi-junction solar cells under variable voltage bias", 2010 35th IEEE Photovoltaic Specialists Conference, pp. 000712-000716, DOI: 10.1109/PVSC .2010.5617046). The device was used to measure the external quantum performance spectra of a triple-junction InGaP/InGaAs/Ge solar cell. The solar cell is simultaneously illuminated by three different light beams. Two intense laser beams (red and blue) strongly excite the InGaP and Ge p-n junctions, so the total photocurrent flowing through the device is limited by the InGaAs junction, which is illuminated through the monochromator by a lower-intensity xenon lamp beam. This fiber is mechanically sheared, and the photocurrent is recorded by a synchronous detector. In this experimental geometry, the mid-InGaAs junction characteristics are measurable; in order to carry out such studies with the remaining two p-n junctions, it is necessary to select the intensities of light fibers of different wavelengths corresponding to the optical absorption region of the junctions accordingly.

The disadvantage of a known characterization device is the inevitable temperature change of the measuring device under the influence of high-intensity light beams, designed to eliminate the influence of non-measurable p-n junctions. That temperature change can significantly influence both the characteristics of the tested junction and the electrical parameters of other boundaries between different material layers in the DSSE formation. In addition, the measurement results can be affected by the luminescence radiation from strongly excited junctions with a gap of reserve energies larger than that of the investigated junction.

It is possible to study the electrical and optical characteristics of semiconductors and devices made from them without the use of electrical contacts using short pulses of terahertz (THz) radiation. The US patent US 9,450,536 B2 (2016. 09. 20) prototype describes the method of characterization of semiconductor optoelectronic devices using such pulses and the device that implements it. The device consists of a pulsed laser that allows the generation of three optical pulses of different wavelengths and is used to illuminate the investigated DSSE, the part of the time delay of those pulses required to separate the THz radiation pulses generated by each of them, and the THz pulse detection and result processing parts. The method is based on the phenomenon of generation of THz radiation pulses in a semiconductor illuminated by a short optical pulse. Light of different wavelengths is absorbed in different DSSE layers, and the electrons and holes created due to this absorption will be affected by the internal electric field in the corresponding layer and separated in space from each other. The resulting time-varying electric dipole due to this splitting is the main reason for the appearance of the radiated THz pulse. From the amplitude of the THz pulse, it is possible to estimate the strength of the internal electric field in the individual layers of materials that make up the DSSE formation. In this way, with the help of the THz pulse generation beam focused and scanning the surface of the DSSE device, it is also possible to evaluate material defects and morphological changes located at different depths and in different places of the device, which can determine the non-radiative recombination of charge carriers and changes in the internal electric field.

In the mentioned patent, several devices capable of realizing this method of characterization are proposed. In the first of them, the source of optical pulses is a femtosecond laser; next to the main laser wavelength pulse, second and third harmonic laser pulses are used, obtained by passing the main pulse through the corresponding nonlinear optical crystals. The second and third harmonic pulses in the delay lines are time-locked with respect to the fundamental harmonic pulse so that the detector can record the amplitudes of each of the THz pulses they generate separately.

In the second version of the device proposed in the aforementioned US patent, a "white", supercontinuum-generating femtosecond laser with a very wide pulse spectrum is selected as the source of optical pulses. Filters select three different parts of this spectrum; they will make it possible to obtain optical pulses of different wavelengths (and longer than the original duration), which will be delayed and used to illuminate the DSSE in a similar way as it is done in the case of the first embodiment.

Both of these options are difficult to implement. The AMO Solar spectrum covers a wide range of wavelengths from ~300 nm to 1600 nm, we can expect the largest contribution to the generation of electron-hole pairs in the DSSE in the northern part, where we will find the most photons. From the distribution of the number of photons with various energies in the spectrum of the Solar AMO, it can be seen that the most technically interesting part of this spectrum is the part of the spectrum where photon energies are between 1 eV and 3 eV. Therefore, regarding the first of the options proposed in the prototype, in which the measurement uses the first harmonic of the femtosecond laser and the pulses of its doubled and tripled frequency, the most optimal would be a laser system generating light quanta of 1 eV energy. Such systems exist on the market and are based on active media doped with Yb or Nd ions. The Ti:sapphire laser discussed in the US patent - the most widespread of all femtosecond lasers - will not work, because its quantum energy is ~1.5 eV, so the energy of triple-frequency photons will reach 4.5 eV - a part of the spectrum that is not relevant from the point of view of solar energy.

The spectrum of femtosecond pulses of "white" light can span from ~200 nm to more than 1600 nm, but such pulses are obtained by passing very powerful initial, contagion optical pulses through one or another nonlinear optical medium: gas or liquid (R L. Fork, et al, "Femtosecond white-light continuum pulses", Opt. Lett., 8, pp. 1-3 (1983), DOI:10.1364/OL.8.000001) or a solid such as amorphous quartz (A. Saliminia et al. al., "Ultra-broad and coherent white light generation in silica glass by focused femtosecond pulses at 1.5 pm", Opt.Express, 13, pp. 57315738 (2005), DOI:10.1364/OPEX.13.005731). For this reason, the resulting spectrum shape will be strongly influenced by the optical resonances of the nonlinear medium, and the intensity of various parts of the spectrum will vary greatly.

Another common shortcoming of the method and device realizations proposed in the prototype is related to the use of a sequence of multiple optical pulses, in which the distances between the pulses are greater than the duration of the THz pulse generation. The latter value is about 1 ps, but the lifetime of charge carriers in solar cells is usually several orders of magnitude longer, so electrons without recombination or removal of j contacts will cause the absorption of THz pulses and distort the measurement results.

TECHNICAL PROBLEM SOLVED

The present invention aims to extend the measurable range of wavelengths and increase the accuracy of spectral measurements and to create an easily realized device for measuring the performance characteristics of various optoelectronic components, including multi-junction solar cells.

THE ESSENCE OF SOLVING THE PROBLEM

The essence of the solution to the problem is achieved by the characterization device for semiconductor optoelectronic devices proposed by the invention, which includes a femtosecond laser, a semi-transparent mirror, dividing the laser beam into the first part of the beam and the second part of the beam, which has a higher intensity than the first part of the beam and is directed to the test device. a line changing the optical path of the first part of the beam, a terahertz pulse detector, an activatable first part of the beam and a recording means, where the device has an optical parametric amplifier, which is placed in the optical path of the second part of the laser beam between the semi-transparent mirror and the test device, where the second higher intensity femtosecond laser part of the beam enters an optical parametric amplifier, which creates a beam of variable-wavelength femtosecond light pulses, which is directed at an angle to the surface of the test device, where the terahertz (THz) radiation pulses generated and reflected from the test device enter the detector, which is activated by the first pass through the delay line part of the fiber, and the THz radiation pulses registered in the detector enter the recording device, which determines the spectral characteristic of the test device according to the time shape of the THz pulses registered in the detector.

UTILITY OF THE INVENTION

The spectral response of the test optoelectronic device is determined by measuring the shape of the THz pulses generated in the test device illuminated with pulses from an optical parametric amplifier (OPS) of various wavelengths. The collected data can be analyzed by integrating the shape of THz pulses in time, or by means of computer modeling of charge carrier dynamics and THz generation in the device. The wavelength of pulses generated by OPS can be changed in a wide spectral range in small steps, so its use allows to expand the range of determining the spectral characteristic and increase the accuracy of its measurement.

OPS pulses of different wavelengths will be absorbed at different depths of the research device, for example, DSSE - in those layers, the gap between the reserve energies of the material will be smaller than the quantum energy of the OPS pulse. The femtosecond optical pulse will create electrons and holes in the corresponding layer, which, moving in the internal electric field in the layer, will be spatially separated from each other and shield it. This local photocurrent J will change in time, creating a time-varying electric dipole, which will cause the generation of an electromagnetic radiation pulse. Since the maximum variation of the photocurrent will take place during the duration of the optical pulse, the duration of the entire generated electromagnetic pulse will also be several hundred femtoseconds, and the spectrum of the pulse itself will correspond to the THz frequency range. Pulses of such frequencies will propagate in space quasi-optically, so their parameters will be able to be measured without using any electrical contacts, and the proposed device itself will be easily implemented for measuring performance characteristics of various optoelectronic components.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings presented do not limit the scope of the invention.

Fig. 1. Schematic diagram of the proposed device.

Fig. 2. Time variation of exciting optical pulse LP4 and photocurrent pulse J.

Fig. 3. Time variation of the terahertz pulse amplitude.

Fig. 4. Dependence of the duration of the pulses generated by the optical parametric amplifier on the radiation wavelength.

Fig. 5. THz emission spectra of P-l-N diodes with i-regions from GaAsBi (triangles), GaAs (black circles) and AIGaAs (squares) normalized to a constant number of photons in a light pulse.

Fig. 6. The THz emission spectra of the tandem solar cell are normalized to a constant number of photons in the light pulse.

DESCRIPTION OF IMPLEMENTATION OF THE INVENTION

The proposed device includes a femtosecond laser 1 that generates light pulses 2. In a semi-transparent mirror 3, the light pulse beam 2 is split into two parts: the first part 4, which is of lower intensity and is used to activate the THz radiation pulse detector 5, and the second part of higher intensity part 6 entering the optical parametric amplifier (OPS) 7. OPS 7 generates light pulses 8 of variable wavelength femtosecond duration, which are used to excite the test device 9. The light pulses 8 fall at an angle of 45° to the test device 9; in their reflection path there is a detector 5, which registers the THz radiation pulses 10 generated due to the absorption of femtosecond light pulses in the test device 9. The detector 5 can be a photoconductive antenna made of a semiconductor sensitive to the wavelength of fiber 4 with charge carrier lifetimes shorter than 1 ps, or an electro-optic Pockels effect a crystal of the containing substance. The first part of the beam 4 is directed to the detector 5 through the mirror 11 and the delay line 12. The THz radiation pulses registered in the detector enter the recording device 13, which determines the spectral characteristic of the test device 9 according to the time shape of the THz pulses registered in the detector.

Depending on the wavelength of the fiber 8 that passed through the OPS 7, it will be absorbed in different places of the optoelectronic device 9 under investigation - in those places where the gap between the reserve energies of the material will be lower than the energy of the pulse photons of the fiber 8. If there is an internal electric field in the corresponding part, the unbalanced charge carriers created during light absorption will create a pulse of photocurrent J(t) that changes rapidly in time. This photocurrent will induce a pulse of electromagnetic radiation with characteristic frequencies in the THz range, so it will be able to propagate in space quasi-optically, and its amplitude will be proportional to the time derivative of the photocurrent ETHz~dJ(t)/dt.

The time variations of fiber 8 and photocurrent J are shown in Fig. 2. The time variation of Ethz is shown in Fig. 3. As can be seen from these figures, the amplitude of the THz pulse 10 reaches its maximum during the duration of the optical pulse, when the variation of the photocurrent with time is the largest. In addition, this amplitude will depend on the duration of the optical pulse. Meanwhile, the position of the photocurrent J amplitude maximum in time corresponds to the duration of the THz pulse 10, and its value, which can be determined by time integration Ετηζ(ϊ), will depend significantly less on the duration of the optical pulse. The photocurrent generated when illuminating the structure under investigation with radiation pulses of various wavelengths will depend on the absorption coefficient and the strength of the internal electric field, and its variation over time will depend on the structural factors of the material determining the recombination of charge carriers in a specific location of the device structure.

BEST IMPLEMENTATION OPTIONS

For the implementation of the invention, a device based on an enhanced ytterbium-doped potassium and gadolinium tungstate Yb:KGW laser 1 (PHAROS, Light conversion) was developed, which generated pulses with a wavelength of 1030 nm, a duration of 160 fs and a repetition rate of 200 kHz. Laser 1 beam 2 was directed to an optical parametric amplifier OPS 7 (ORPHEUS, Light Conversion), which was converted to generate optical femtosecond pulses with wavelengths varying from 640 nm to 2600 nm. Shorter wavelengths of up to 400 nm were obtained by generating second harmonic pulses in a beta barium borate (BBO) crystal. The optical pulses fell at an angle of 45° to the test device 9, and the THz radiation pulse 10 was observed in the reflection direction using a GaAsBi photoconductive antenna (UAB Teravil), which was activated by a part 4 of the Yb:KGW laser beam illuminating the detector 5 after passing through a variable-length delay line 11. Fig. 4 shows the duration of pulses of various wavelengths generated by OPS 7 determined by second harmonic autocorrelation. This illustration shows that this parameter varies little over the entire range; its greater change is observed only when passing from signal to side waves generated by OPS.

Measurements of the spectral characteristics of four devices were performed: three single-junction P-l-N diodes with active absorption layers made of different materials - GaAs, GaAIAs and GaAsBi, and a two-junction photovoltaic cell: GaAs and AIGaAs. All these samples were grown in a Veeco molecular fiber epitaxy on GaAs substrates.

The thickness of the active absorption region of the GaAs diode was 2000 nm. The thickness of the AIGaAs diode absorption region was equal to 450 nm, the proportion of Al in the trinary AIGaAs compound was 33%. In the third diode, a 500 nm thick GaAsBi (with 4% Bi) i-layer was sandwiched between the contact n- and p-type GaAs layers. The structure of the layers of the studied double junction solar cell is shown in the table below.

Layer Material Thickness n m Doping cm-3 clarifying optical structure _SiO2 60 - clarifying optical structure _HfO2 30 - clarifying optical structure _SiO2 10 - window-n ^'θ.85θ^θ.15^ ^δ 30 4e18 emitter-n ^'o.33^ ^a O.67^ ^S 100 1e18 base-p ^'o.33^ ^a O.67^ ^S 700 1e17 rear field -p ^'o.9O^ ^a O.1O^ ^S 200 5e18 tunnel junction-p GaAs 20 5e19 tunnel junction-n GaAs 50 2e19 window-n ^'o.9O^ ^a O.1O^ ^S 200 4e18 emitter-n GaAs 100 1e18 base-p GaAs 1800 1e17 rear field -p ^'o.2O^ ^a O.8O^ ^S 150 2e18 buffer-p GaAs 200 4e18 tray-p GaAs

The measurement results of all three diodes are shown in Fig. 5. During the measurement, the test sample was illuminated with a 6 mW average power, S polarization beam falling at an angle of 45° to the surface normal of the test device (9). In all three cases, the emission of THz pulses started when the photon energy of the optical pulse exceeded the reserve energy gap of the material of the active region of the diode. Fig. The THz pulse amplitude values shown in Fig. 5 are normalized to a constant number of photons and therefore allow an accurate comparison of the different test specimens. The spectra of GaAs and AIGaAs diodes are significantly smoother than that of GaAsBi. In the latter region of photon energies higher than the reserve gap, strong THz pulse amplitude oscillations are observed, which can be explained by light interference effects due to reflections from the GaAs substrate, which is transparent to the radiation of this spectral range.

The results of the research of a tandem solar cell consisting of two subcells: Alo.33Gao.67As (band gap s _g is equal to 1.85 eV) and GaAs (e _g =1.42 eV) are shown in Fig. 6. In this picture, it is possible to clearly distinguish the absorption regions of both parts of the tandem solar cell and the different amplitudes of the photocurrent. A lower generated photocurrent in the GaAs subcell than in the AIGaAs subcell is clearly visible. This result is consistent with measurements of the external quantum efficiency (EQE) and simulation results of the reflection spectrum of the device. The EQE measurement shows a lower quantum efficiency of the GaAs subcell, while the reflectance spectrum shows a significantly higher reflectance in the region where the light is absorbed by the GaAs subcell. Therefore, the lower absorption of the GaAs layer results in a lower photocurrent amplitude in THz pulse-based measurements.

Claims (1)

Device for measuring the spectral characteristics of semiconductor optoelectronic devices, including - a femtosecond laser (1); - a semi-transparent mirror (3) dividing the beam (2) of the laser (1) into the first part of the beam (4) and the second part of the beam (6), which has a higher intensity than the first part of the beam (4) and is directed to the test device ( 9); - a delay line (12) changing the optical path of the first part of the fiber (4), - a terahertz pulse detector (5), which is activated by the first part of the fiber (4) and - a recording means (13), which differs in that - the device has an optical parametric the amplifier (7), which is placed in the optical path of the second part (6) of the laser beam between the semi-transparent mirror (3) and the test device (9), where the second part (6) of the beam of the higher intensity femtosecond laser (1) enters the optical parametric amplifier ( 7), creating a beam of femtosecond light pulses (8) of variable wavelength, which is directed at an angle to the surface of the test device (9), where the terahertz (THz) radiation pulses (10) generated in and reflected from the test device enter the detector (5), which is activated by the first part of the fiber (4) that has passed through the delay line (12), and the THz radiation pulses registered in the detector enter the recording means (13), which determines the spectral characteristic of the test device (9) according to the time shape of the THz pulses registered in the detector.