OSA's Digital Library

Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 11 — May. 30, 2005
  • pp: 3951–3960
« Show journal navigation

Practical photoluminescence and photoreflectance spectroscopic system for optical characterization of semiconductor devices

Ching-Hwa Ho, Kuo-Wei Huang, Yu-Shyan Lin, and Der-Yuh Lin  »View Author Affiliations


Optics Express, Vol. 13, Issue 11, pp. 3951-3960 (2005)
http://dx.doi.org/10.1364/OPEX.13.003951


View Full Text Article

Acrobat PDF (150 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We present a practical experimental design for performing photoluminescence (PL) and photoreflectance (PR) measurements of semiconductors with only one PL spectroscopic system. The measurement setup is more cost efficient than typical PL-plus-PR systems. The design of the experimental setup of the PL–PR system is described in detail. Measurements of two actual device structures, a high-electron-mobility transistor (HEMT) and a double heterojunction-bipolar transistor (DHBT), are carried out by using this design. The experimental PL and PR spectra of the HEMT device, as well as polarized-photoreflectance (PPR) spectra of the DHBT structure, are analyzed in detailed and discussed. The experimental analyses demonstrate the well-behaved performance of this PL–PR design.

© 2005 Optical Society of America

1. Introduction

In this paper we present a practical design for integrating PL and PR spectral measurements in one PL spectroscopic system. The measurement setup is cost efficient compared with the usual PL plus PR systems. In addition this PL–PR design has an enhanced ability to increase the signal-to-noise (S/N) ratio of signal detection when comparing with conventional PR experiments. The design of the experimental setup for the PL–PR system is described. Measurements of a GaAs/InGaAs grading-channel HEMT and an InGaP/InGaAsN/GaAs double heterojunction-bipolar transistor (DHBT) are carried out with this measurement design. The experimental PL and PR spectra of the HEMT device as well as the polarized-photoreflectance (PPR) spectra of the DHBT structure are analyzed and discussed. The intersubband energies, Fermi-level energy, and sheet carrier density of a two-dimensional-electron gas (2DEG) and the built-in electric field of the graded-channel HEMT are evaluated. The interband transitions and built-in electric fields of the collector-base and emitter-base junctions for the DHBT device are characterized. Well-resolved and easily analyzed properties of the experimental spectra for the HEMT and DHBT devices demonstrate the well-behaved performance of this PL–PR design.

2. Measurement design

A representative scheme of the experimental setup of the PL–PR measurement system is shown in Fig. 1. A Triax 320 imaging spectrometer equipped with three gratings of 600, 1200, and 2400 groves/mm acted as the optical dispersion unit. Two detecting elements, a photomultiplier tube (PMT) and a (TE) thermoelectric-cooled InGaAs detector were attached at the outside of two exit slits. The PMT and TE-cooled InGaAs photodetectors cover a wide measured spectral range, from 190 to 1650 nm. The operating temperature of the TE-cooled InGaAs detector was controlled by a TE-cooled driver with maximum cooling of -50°C. A data acquisition (DAQ) unit in the PL–PR system is capable of integrating the experimental details, such as implementing the analog-to-digital (A/D) conversion of signal detection from the PMT or the InGaAs detector, supplying high voltage to the PMT, and transferring the digital data to a personal computer (PC). The DAQ unit communicated with the PC via an RS-232 bus. To improve the S/N ratio of signal detection, ac phase-sensitive detection (PSD) is implemented by using a lock-in amplifier and an optical chopper. The chopper is utilized to cut the laser light into an ac-type pumping beam for both PL and PR measurements and to provide a reference signal for the chopping frequency for the lock-in amplifier. For PL measurements of the HEMT device, a frequency-doubled Nd–YAG laser (peak wavelength λp =532 nm) with an average output power of 100 mW was used as the pump light source. The PL emissions from the sample were collected and focused onto the spectrometer via a planar–convex lens. The PL measurements were performed by programming the control of the spectrometer and DAQ unit via the RS-232 bus connections.

Fig. 1. Experimental setup of the PL–PR system used for PL and PR measurements of semiconductors.

For PR measurements, a dc 3-V tungsten halogen lamp acted as the white-light source. The white light was focused onto the sample via a planar–convex lens. The reflected light from the sample was collected and focused onto an imaging spectrometer by another planar–convex lens. A He–Ne laser (λp =632.8 nm) together with a neutral-density filter (ND 2.0) acted as the modulation light source of the HEMT sample. A 532-nm Nd–YAG laser combined with an ND 3.0 (0.1%) neutral-density filter was used for modulation of the DHBT device. A TE-cooled InGaAs detector operated at ~0°C was utilized for optical detection. A pair of visible-dichroic-sheet polarizers was utilized to perform the polarization-dependent measurements of the DHBT. The PR spectral measurements were implemented by first measuring and recording the spectral data of the white-light source reflected from the sample surface (i.e., R) and then detecting the change in reflection of the sample (i.e., ΔR) after photo perturbations. Note that this PL–PR system can avoid background-light interference arising from the pump laser because of the dispersion property of the imaging spectrometer, which prevents detection of the laser wavelength. It has an enhanced S/N ratio of signal detection with respect to a conventional PR system [4

4. F. H. Pollak and H. Shen, “Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,” Mater. Sci. Eng. R10, 275–374 (1993).

].

3. Experimental results and discussion

The functional performance of the PL–PR system design was tested by using two selected samples of a graded-channel GaAs/InxGa1−xAs HEMT and an InGaP/InGaAsN/GaAs DHBT at 300 K. Sample specifications and measurement conditions for testing the PL–PR design are summarized in Table 1. The graded-channel HEMT (sample 1) was grown by a computer-controlled LP-MOCVD [9

9. W. C. Hsu, C. M. Chen, and R. T. Hsu, “A δ-doped GaAs/graded InxGa1-xAs pseudomorphic structure grown by low-pressure metal organic chemical vapor deposition,” Appl. Phys. Lett. 59, 1075–1077 (1991). [CrossRef]

]. The epilayers of sample 1 were grown on a semi-insulating (100)-oriented GaAs substrate and were followed by an undoped 1 µm GaAs buffer layer, a 90 Å undoped graded InxGa1−xAs layer, an 80 Å undoped GaAs layer, a δ-doped GaAs layer, and finally a 400 Å undoped GaAs cap layer. The In composition of the channel layer in sample 1 was varied from x=0.15 to x=0.25. Figure 2 shows the representative energy band scheme of the graded-channel HEMT. A 2DEG was formed in the channel layer as a result of the electrons’ spilling over the band discontinuity from the N+δ-doped GaAs layer into the InxGa1−xAs grading channel, which also shifted the Fermi level above the conduction band edge inside the channel well.

Table 1. Specifications and Measurement Conditions of the Testing Samples Used in the PL and PR Experiments

table-icon
View This Table
| View All Tables
Fig. 2. Representative energy band diagram of a selective sample of InxGa1−xAs/GaAs graded-channel HEMT.
Fig. 3. Experimental PR and PL spectra of the InxGa1−xAs/GaAs graded-channel HEMT device.

ΔRR=α(ε1,ε2)Δε1+β(ε1,ε2)Δε2,
(1)

where α and β are the Seraphin coefficients and Δε 1, Δε 2 are, respectively, the modulated real and imaginary components of the complex dielectric function ε=ε 1+ 2. For bound states, such as the intersubband transitions of a quantum well, it has been shown that electromodulation yields first-derivative spectroscopy [4

4. F. H. Pollak and H. Shen, “Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,” Mater. Sci. Eng. R10, 275–374 (1993).

]. In the presence of a sufficiently dense 2DEG, the intersubband absorption function will be a broadened two-dimensional density of states multiplied by a Fermi level-filling function. From the one-electron theory, the imaginary part of the dielectric function can be expressed as [11

11. Y. Yin, H. Qiang, F. H. Pollak, D. C. Streit, and M. Wojtowicz, “Two-dimensional electron gas effects in the electromodulation spectra of a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation-doped quantum well structure,” Appl. Phys. Lett. 61, 1579–1581 (1992). [CrossRef]

]

ε2=jDj{Im[ln(EEj(mn)+iΓj)]}·(1fej),
(2)

where Dj is the amplitude of the jth feature, E is the photon energy, Γ j is the broadening parameter, and Ej (mn) is the inter-subband energy given by Ej (mn)=Em,jCEn,jV. Em,jC and En,jV are the energies of the mth conduction and nth valence subband, respectively, referred to the jth feature. The Fermi function in Eq. (2) can be expressed as

fej={1+exp[((λEλEj(mn)E¯j(m))kT]}1,
(3)

with

E¯j(m)=EFEm,jC,
(4)

where EF is the Fermi energy. The parameter λ in Eq. (3) is given by

λ=mh*me*+mh*,
(5)

where me* and mh* are the electron and the in-plane heavy-hole effective masses, respectively, in units of the free electron mass.

The intersubband transitions of the PR spectrum in Fig. 3 can be analyzed by fitting the PR spectrum to Eqs. (1) and (2). The effective masses of the electron and in-plane heavy hole for the channel layer are assumed to be 0.058 and 0.326. The fitting parameters are Dj, Ej (mn), Γ j , and Ē j (m), respectively. The obtained intersubband energies of the well transitions Ej (mn) in Fig. 3 are indicated with arrows and are listed in Table 2. It must be noted that the line shapes of the PR spectrum of the graded-channel HEMT are uncommon for modulation spectroscopy from a quantum-well system in which 11H is usually the dominant feature. In Fig. 3 the feature associated with 11H (feature A) is small in comparison with the other transitions. The reasons for the weak 11H transition are the GaAs/InxGa1−xAs conduction band discontinuity and that the δ-doped layer introduces a large density of 2DEG into the InGaAs grading well. The Fermi level is lifted over the first conduction subband, and hence this subband is almost fully occupied by 2DEG. Consequently, the strength of the transition from the first valence subband to the first conduction subband is reduced dramatically, resulting in the weaker 11H feature. More evidence of the 11H feature in the graded-channel HEMT can be observed from the PL spectrum shown in Fig. 3. The maximum luminescent intensity of a quantum well for the PL measurement generally occurs in the ground-state recombination. In the PL spectrum of Fig. 3 the energy location of feature A has nearly the largest spectral amplitude of luminescence, which can be attributed to intersubband recombination from the 11H transition. The luminescence feature located at ~1.3 eV is the 21H transition arising from the recombination of the second conduction subband with the first valence subband (see Fig. 2). The energy value of the Fermi-level location EFE1C can also be determined to be (6±2) meV for the graded-channel HEMT. The sheet density Ns , for the case of a broadened steplike 2D density of electron states [ρ2D(E,Γ)], can be expressed as [11

11. Y. Yin, H. Qiang, F. H. Pollak, D. C. Streit, and M. Wojtowicz, “Two-dimensional electron gas effects in the electromodulation spectra of a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation-doped quantum well structure,” Appl. Phys. Lett. 61, 1579–1581 (1992). [CrossRef]

]

Ns=0ρ2D(E,Γ)·{exp[(EEF)kT]+1}1dE,
(6)

where

ρ2D(E,Γ)=(me*/π2)m{12+(1π)tan1[(EEmC)Γm]}.
(7)

From Eqs. (6) and (7) we can evaluate the sheet density Ns of the graded-channel HEMT and list it in Table 2.

Table 2. Experimental Values of the Intersubband Transition Energies from the InxGa1-xAs Graded-Channel Layer of a HEMT Structure with 2D Sheet Density Ns Obtained by PR Measurement

table-icon
View This Table
| View All Tables

The FKOs in the PR spectrum of Fig. 3 can also be analyzed to yield the built-in electric field at the GaAs interface. The position of the nth extremum in the FKOs can be expressed as [12

12. H. Shen and F. H. Pollak, “Generalized Franz-Keldysh theory of electromodulation,” Phys. Rev. B 42, 7097–7102 (1990). [CrossRef]

]

nπ=(4/3)[(EnE0)Θ]32+χ,
(8)

where En is the photon energy of the nth extremum, E 0 is the bandgap, and χ is an arbitrary phase factor. The electro-optic energy ħΘ is given by

(Θ)3=q22F22μ,
(9)

where F is the electric field and µ is given by

1μ=1me*+1mh*.
(10)

Here me* and mh* are the effective masses of the electron and the hole, respectively, in units of the free electron mass. The relevant electron and heavy-hole effective masses for GaAs are 0.067 and 0.34, respectively. From the analysis of the FKOs shown in Fig. 3, the built-in electric field of the graded-channel HEMT at the InGaAs/GaAs interface is determined to be F=155±5 kV/cm.

Fig. 4. Structure of the device’s epilayers for an InGaP/InGaAsN/GaAs DHBT sample.
Fig. 5. PPR spectra of the InGaP/InGaAsN/GaAs DHBT device with E‖[110] and E‖[11̄0] polarizations.

4. Conclusions

In conclusion, a practical PL–PR experimental design for performing photoluminescence and photoreflectance measurements of semiconductors by using only one PL spectroscopic system is demonstrated. The functional performance of the measurement design was tested on two semiconductor devices, a GaAs/InGaAs grading-channel HEMT and an InGaP/InGaAsN/GaAs DHBT. From analyses of the PR and PL spectra of the graded-channel HEMT, the intersubband energies, Fermi-level location, and 2D sheet density of the graded InxGa1−xAs channel well were determined. The InGaP/InGaAsN/GaAs DHBT was characterized by using PPR measurements with polarizations along [110] and [11̄0]. The experimental results clearly show two FKOs and some transition features present in the PPR spectra of the DHBT. From analyses of the PPR spectra, the built-in electric fields near the emitter and collector regions are evaluated, and the transition energies in the base, collector, and emitter layers are determined. The well-resolved and easily analyzed properties of the experimental PR and PL spectra of the HEMT and DHBT devices indicate the well-behaved function of this PL–PR design.

Acknowledgments

The authors would like to acknowledge the project fund supported from the National Science Council of Taiwan under the grant No. NSC 93-2215-E-259-002.

References and links

1.

P. Bhattacharya, “Elemental and compound semiconductors,” in Semiconductor Optoelectronic Devices, 2nd ed. (Prentice-Hall, London, 1997), Chap. 1, pp. 2–58.

2.

D. K. Schroder, “Optical characterization,” in Semiconductor Material and Device Characterization1st ed. (Wiley, New York, 1990), Chap. 9, pp. 490–494

3.

M. Cardona, Modulation Spectroscopy (Academic, New York, 1969).

4.

F. H. Pollak and H. Shen, “Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,” Mater. Sci. Eng. R10, 275–374 (1993).

5.

D. E. Aspnes, “Modulation spectroscopy/electric field effects on the dielectric function of semiconductors,” in Handbook on Semiconductors, Vol. 2., M. Balkanski, ed. (North Holland, New York, 1980), p. 109.

6.

C. H. Ho, “Optical study of the structural change in ReS2 single crystals using polarized thermoreflectance spectroscopy,” Opt. Express 13, 8–19 (2005), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-1-8. [CrossRef] [PubMed]

7.

C. H. Ho, P. C. Yen, Y. S. Huang, and K. K. Tiong, “Photoreflectance study of the excitonic transitions of rhenium disulphide layer compounds,” Phys. Rev. B 66, 245207 (2002). [CrossRef]

8.

D. Y. Lin, F. C. Lin, Y. S. Huang, H. Qiang, F. H. Pollak, D. L. Mathine, and G. N. Maracas, “Piezoreflectance and photoreflectance study of GaAs/AlGaAs digital alloy compositional graded structures” J. Appl. Phys. 79, 460–466 (1996). [CrossRef]

9.

W. C. Hsu, C. M. Chen, and R. T. Hsu, “A δ-doped GaAs/graded InxGa1-xAs pseudomorphic structure grown by low-pressure metal organic chemical vapor deposition,” Appl. Phys. Lett. 59, 1075–1077 (1991). [CrossRef]

10.

D. Y. Lin, S. H. Liang, Y. S. Huang, K. K. Tiong, F. H. Pollak, and K. R. Evans, “Room-temperature photoreflectance and photoluminescence characterization of the AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structure with varied quantum well compositional profiles,” J. Appl. Phys. 85, 8235–8241 (1999). [CrossRef]

11.

Y. Yin, H. Qiang, F. H. Pollak, D. C. Streit, and M. Wojtowicz, “Two-dimensional electron gas effects in the electromodulation spectra of a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation-doped quantum well structure,” Appl. Phys. Lett. 61, 1579–1581 (1992). [CrossRef]

12.

H. Shen and F. H. Pollak, “Generalized Franz-Keldysh theory of electromodulation,” Phys. Rev. B 42, 7097–7102 (1990). [CrossRef]

13.

C. J. Lin, Y. S. Huang, N. Y. Li, P. W. Li, and K. K. Tiong, “Polarized-photoreflectance characterization of an InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor structure,” J. Appl. Phys. 90, 4565–4569 (2001). [CrossRef]

14.

A. Lindell, M. Pessa, A. Salokatve, F. Bernardini, and M. Paalanen, “Band offsets at the GaInP/GaAs heterojunction,” J. Appl. Phys. 82, 3374–3380 (1997). [CrossRef]

OCIS Codes
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(300.6380) Spectroscopy : Spectroscopy, modulation
(300.6470) Spectroscopy : Spectroscopy, semiconductors

ToC Category:
Research Papers

History
Original Manuscript: February 17, 2005
Revised Manuscript: May 12, 2005
Published: May 30, 2005

Citation
Ching-Hwa Ho, Kuo-Wei Huang, Yu-Shyan Lin, and Der-Yuh Lin, "Practical photoluminescence and photoreflectance spectroscopic system for optical characterization of semiconductor devices," Opt. Express 13, 3951-3960 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-11-3951


Sort:  Journal  |  Reset  

References

  1. P. Bhattacharya, �??Elemental and compound semiconductors,�?? in Semiconductor Optoelectronic Devices, 2nd ed. (Prentice-Hall, London, 1997), Chap. 1, pp. 2�??58.
  2. D. K. Schroder, �??Optical characterization,�?? in Semiconductor Material and Device Characterization 1st ed. (Wiley, New York, 1990), Chap. 9, pp. 490�??494
  3. M. Cardona, Modulation Spectroscopy (Academic, New York, 1969).
  4. F. H. Pollak and H. Shen, �??Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,�?? Mater. Sci. Eng. R10, 275�??374 (1993).
  5. D. E. Aspnes, �??Modulation spectroscopy/electric field effects on the dielectric function of semiconductors,�?? in Handbook on Semiconductors, Vol. 2., M. Balkanski, ed. (North Holland, New York, 1980), p. 109.
  6. C. H. Ho, �??Optical study of the structural change in ReS2 single crystals using polarized thermoreflectance spectroscopy,�?? Opt. Express 13, 8�??19 (2005), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-1-8.">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-1-8</a> [CrossRef] [PubMed]
  7. C. H. Ho, P. C. Yen, Y. S. Huang, and K. K. Tiong, �??Photoreflectance study of the excitonic transitions of rhenium disulphide layer compounds,�?? Phys. Rev. B 66, 245207 (2002). [CrossRef]
  8. D. Y. Lin, F. C. Lin, Y. S. Huang, H. Qiang, F. H. Pollak, D. L. Mathine, and G. N. Maracas, �??Piezoreflectance and photoreflectance study of GaAs/AlGaAs digital alloy compositional graded structures�?? J. Appl. Phys. 79, 460�??466 (1996). [CrossRef]
  9. W. C. Hsu, C. M. Chen, and R. T. Hsu, �??A δ-doped GaAs/graded InxGa1�??xAs pseudomorphic structure grown by low-pressure metal organic chemical vapor deposition,�?? Appl. Phys. Lett. 59, 1075�??1077 (1991). [CrossRef]
  10. D. Y. Lin, S. H. Liang, Y. S. Huang, K. K. Tiong, F. H. Pollak, and K. R. Evans, �??Room-temperature photoreflectance and photoluminescence characterization of the AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structure with varied quantum well compositional profiles,�?? J. Appl. Phys. 85, 8235�??8241 (1999). [CrossRef]
  11. Y. Yin, H. Qiang, F. H. Pollak, D. C. Streit, and M. Wojtowicz, �??Two-dimensional electron gas effects in the electromodulation spectra of a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation-doped quantum well structure,�?? Appl. Phys. Lett. 61, 1579�??1581 (1992). [CrossRef]
  12. H. Shen and F. H. Pollak, �??Generalized Franz�??Keldysh theory of electromodulation,�?? Phys. Rev. B 42, 7097�??7102 (1990). [CrossRef]
  13. C. J. Lin, Y. S. Huang, N. Y. Li, P. W. Li, and K. K. Tiong, �??Polarized-photoreflectance characterization of an InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor structure,�?? J. Appl. Phys. 90, 4565�??4569 (2001). [CrossRef]
  14. A. Lindell, M. Pessa, A. Salokatve, F. Bernardini, and M. Paalanen, �??Band offsets at the GaInP/GaAs heterojunction,�?? J. Appl. Phys. 82, 3374�??3380 (1997). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited