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Optical determination of carrier density in pseudomorphic AlGaAs/InGaAs/GaAs

hetero‐field‐effect transistor structures by photoluminescence

H. Brugger , H. Müssig, C. Wölk, K. Kern, and D. Heitmann 

Citation: Applied Physics Letters 59, 2739 (1991); doi: 10.1063/1.105904 

View online: http://dx.doi.org/10.1063/1.105904 

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/59/21?ver=pdfcov 

Published by the AIP Publishing 

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Optical determination of carrier density in pseudomorphic

AIGaAs/lnGaAs/GaAs hetero-field-effect transistor structures

by photoluminescence

H. Brugger, H. MUssig, and C. W6lk

Daimler Benz AG, Research Center Urn, D-7900 Urn, P.0. Box 2360, Germany

K. Kern and D. Heitmann

Max-Plan&-Institut ftir Festkiirperjbrschung, D-7000 Stuttgart 80, Germany

(Received 5 July 1991; accepted for publication 27 August 199 1)

A photoluminescence (PL) analysis of a highly degenerate two-dimensional electron gas

(2DEG) in pseudomorphic modulation-doped AlGaAs/InGaAsiGaAs transistor structures is

reported. The PL response from samples with one or two populated electron subbands is

dominated by one or two spectral bands, respectively, with a high-energy intensity cutoff. The

spectral width varies linearly with the measured 2DEG sheet density

n,

or with a

Schottky barrier depletion voltage, which directly reflects the two-dimensional density of

states (2DDOS) below the Fermi level. Wetused the effective electron mass from cyclotron

resonance experiments to evaluate the 2DDOS and can thus directly determine it, from

the spectral width via the 2DDOS. Independent ~1,values were obtained from Shubnikov-de

Haas measurements and agree excellently with n,Y alues from PL.

Heterojunction field-effect transistors (HFETs) on the

modulation-doped AlGaAs/GaAs material system have

demonstrated their high potential for low noise amplifiers.’

Improved device performance in the microwave and mm

wave frequency region is achieved by incorporation of a

pseudomorphic (PM) InGaAs quantum well (QW) .53

This advance arises in part as a consequence of the higher

achievable two-dimensional electron gas (2DEG) density

n, and the better confinement of the carriers in the channel

due to the larger energy discontinuity in the conduction

band. Information about the material parameter n, is usu-

ally derived from magneto-transport measurements, .e.g.,

Shubnikov-de Haas (SdH) or Hall technique. However,

separately fabricated samples with mesa bars and electrical

contacts are necessary. In addition, the parallel conduction

in the highly doped cap layers complicates the evaluation

of n, especially from Hall experiments.

Photoluminescence (PL) spectroscopy has been used

successfully to determine the impurity concentration in

bulk epitaxial materia14’

and to investigate the plasma be-

havior and many body effects of a highly degenerate two-

dimensional carrier system in n-type modulation doped

structures.“g In the AlGaAs/InGaAs/GaAs HFET sys-

tem both the 2DEG and the photogenerated holes are con-

fined in the InGaAs QW, which greatly enhances the PL

efficiency due to the strong wave function overlap of elec-

trons and holes. Additionally, due to the lower energy gap

of InGaAs the PL transitions are energetically separated

from transitions of adjacent layers. This allows a con-

venient light detection of the 2DEG emission spectrum.

Our experiments on AlGaAs/GaAs HFET structures

without an InGaAs QW have shown, that very intense

GaAs recombination lines from the buffer and the highly

doped cap layers cover almost all of the interesting 2DEG

spectrum.

In this letter we present the results from low-temper-

ature low excitation power PL investigations of AlGaAs/

InGaAs/GaAs HFET structures with various In concen-

trations (x),

well thicknesses (L,) and doping

concentrations in the AlGaAs layer. We observe strong PL

signals, which reflect the whole energy spectrum of the

2DEG. The spectral width of the observed PL depends

linearly on n, This relationship allows a determination of

II, via the two-dimensional density of states (2DDOS),

which is evaluated with the effective electron mass deter-

mined by cyclotron resonance (CR) experiments. The op-

tical method allows a nondestructive determination of

n,

on wafers for device fabrication in a fast and contactless

way. To substantiate our results,. SdH measurements were

carried out on the same samples, and the n, values obtained

are in good agreement with n, values from PL.

The HFET layers used for this study were grown by

molecular beam epitaxy (MBE) on GaAs substrates. The

epitaxial layer sequence consists of: undoped 0.9 ,um thick

GaAs buffer layer, undoped In,Gai _ XAs QW with differ-

ent x and

L,

(see Table I), spacer layer, highly Si-doped

Al,,,Ga,,,As layer, and a final GaAs cap. Details about

the sample growth and device results are published

elsewhere. lo

PL spectra were recorded with a Fourier

Transform Spectrometer and a liquid nitrogen cooled Ge

detector. Data were taken under excitation of an Ar ’ laser

(488 nm) with a power density of a few mW/cm’. Samples

were mounted in a closed-cycle He cryostat with a heating

facility ( 10-320 K) and a large area viewport. This allows

rapid cooling cycles and PL scanning experiments on two-

inch wafers.”

Figure 1 exhibits typical PL spectra of pseudomorphic

HFET structures with different L, x, and n,. PL from the

InGaAs layer appears in the energy region between 1.15

and 1.4 eV. It consists of one or. two spectral bands arising

from recombinations of 2DEG from the n = 1 and n = 2

subbands, respectively, with holes in the n = 1 heavy-hole

subband. To assign the experimental transitions we have

calculated the electron and hole subband structure and

2739

Appl. Phys. Lett. 59 (21), 18 November 1991

0003~6951/91/462739-03$02.00

@ 1991 American Institute of Physics

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TABLE I. QW parameters of PM HFETs, n, values from SdH and PL, energies from PL and from calculation.

Sample

In,Ga, -As QW

SdH n,( 10” cm ‘)

PL (meV)

No.

4%)

LAnd

1

4

2

%

Ef-@~ AI?’

la

18

8

1.10

0

40

2=

18

12

1.16

0

78b

44

3"

18

16

1.52

0

54

52

4

18

12

1.90

0.20

63

70

5

27

12

2.25

0.34

70

82

6

18

7

1.96

0

68

7

18

7

2.15

0

75

8

18

12

1.90

0

65

65

9

14'

17

1.90

0.40

54

67

%amples with an Ala,,Gao.6,As/ InGaAs/Al~,~~G~.~,As layer sequence.

b@ transition identified at higher sample temperature and/or excitation power density.

‘In content determined by x-ray analysis.

PL n,( 10” cm - “)

1

n,

nz

1.18

0

1.30

0

1.54

0

2.08

0.21

2.43

0.36

2.02

0

2.22

0

1.93

0

1.99

0.39

Calculation (meV) ~=

----

4 - -6

EF- IT:,

-

141

37

77

39

54

51

65

64 .I

73

80 -

67

73 =

65

63

54

66

wave functions by solving the Schriidinger and Poisson

equation self-consistently including strain and nonparabo-

licity. The x value is a fitting parameter and the total

2DEG n, is used from SdH data (Table I). The arrows

mark the expected subband transitions (E,,E,) and the

Fermi energy EF. The modulation doping causes a strong

band-bending potential, which yields to a strong modifica-

tion of wave functions and energy levels in the InGaAs

QW compared to an undoped QW. Hence the E, transition

is greatly enhanced due to the larger electron hole overlap

integral.

In all samples investigated, the PL intensity increases

on the low energy side of the E, band within S-10 meV

(half FWHM) to peak maximum. On the high energy side

the spectra were found to fall into three qualitatively dif-

ferent categories depending on the energetic position of EF

relative to E2 (see Fig. 1) :

( 1) E,(E,: The PL spectrum is dominated by one

spectral band (E,) which broadens significantly on the

high energy side and shows a well-defined cutoff at an

energy indicative of the Fermi-level (sample Nos. 1, 2, 6,

7).

(2) EFLC;E~:The emission from the El band merges

into intense and strongly excitation power dependent re-

combinations from the n = 2 subband. Both, an enhanced

intensity feature close to EF and a symmetric, excitonlike

transition from the n = 2 subband (E,) is observed (sam-

ple Nos. 3, 8). In samples where EF is several kT below E2

we found a clear separation of the two features similar to

FIG. 1. Typical PL spectra from PM HFET structures with different

In,Ga, -As, QWs and n,. The arrows mark the calculated values for E,,

E,, and Ep

the observations of Colvard et (11.~’ concomitant with a

drop of the E2 intensity due to the smaller thermal popu-

lation.

(3) EF> E2: The PL emission is dominated by two

spectral bands (E1,E2) originating from the 2DEG in the

n = 1 and n = 2 subbands. Both bands show an asymmet-

ric line shape with a high-energy tail (samples Nos. 4, 5,

9).

Except for the recombination features close to the ex-

pected EF position in case 2, we found no remarkable

change in the PL line shape when the power density is

increased by more than three orders of magnitude froth

< 1 mW/cm” up to 1 W/cm*. The excitonic signal (d,L’)

and the carbon related transition (e,C> originate from the

buffer layer and are typical of high-quality nominally un-

doped MBE-grown GaAs.

In Fig. 2, PL spectra from samples with one and two

occupied electron subbands are drawn on a logarithmic

intensity scale. On the high-energy side of the spectral

Photoluminescence

Shubnikov da Haas

1\41 T=ZOK T=2.2K E ’

=I

_ I

ii’l

G,‘-

.4 x I(l‘cm‘

f

El i

i lx4

**LL 2, 1:;

El*+

,D,

’s 1.2x16-em=

A

4

b,- 0

,*

i+2 %Ij

/I J 1. [A,,

No.2 / 1 \

J LA&/Ei

11,

‘Y” .

n.22

1.3 1.4 0 0.5

1

Energy (ev)

l/B e-'1

FIG. 2. PL spectra and corresponding SdH oscillations from samples with

one (No. 2) and two (No. 9) occupied subbands. The high-energy cutcff

is marked at the crossing of the two dashed lines (No. 9) or at the

intensity enhancement (No. 2).

2740

Appl. Phys. Lett. , Vol. 59, No. 21, 18 November 1991

Brugger eta/.

274.0

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bands a clear intensity cutoff is observed, which is marked

by arrows. On some samples we observe a small intensity

enhancement close to the expected EF position at T = 20

K as seen in Fig. 2 (No. 2) and Fig. 1 (Nos. 1, 3) which

is very temperature sensitive. A similar behavior was found

by Skolnick et aL9 and is attributed to a many body EF

singularity. We define a spectral width AE* as the energy

separation between the high-energy intensity cutoff and the

intensity maximum of the first spectral band marked by

E;“. In all samples investigated we found a correlation be-

tween AE* and n, in good agreement with the 2DEG re-

lation

n,= c nf= [m*(E>/&]AE* c S(EF - Ei) (1)

i

I

here EF - Ei)kT. S = 1 for EF> Ei and S = 0 for

F< Ep m*(E) and i are the electron effective mass and

PL experiments on samples with a semitransparent

chottky gate contact have shown a linear behavior of AE*

ith applied gate voltage which directly reflects the

DDOS below Epto

For depleted structures we found a

L line shape similar to an undoped QW. In that case the

bserved linewidth is strongly influenced by inhomoge-

broadening mechanisms due to well width and alloy

luctuations in the ternary material which are expected to

ontribute significantly to the low-energy onsets of the

in our high-n, HFET structures as we11.12

ally, impurity-assisted processes are also expected

o influence the line shape.13 Therefore, we identify the

of PL intensity (fl and E$) as the onsets of band

although these energies may overestimate the sub-

To confirm the reliability of our n,-values from PL we

ave determined n, independently by SdH measurements

nder perpendicular magnetic field B, and under illumina-

os. 3 and 2. A superposition of two SdH-oscillations is

z

A o”erum.rdDwJpisd (-1

g,, . tmr-omrpied (----4

$0

(E;- E,)

F

/?;gfi

- e/4--y

g40 .?::&F ifti

5

p 20

Gf

L-&Al

50 ?W 1s 2ca

WabaNlmbmdl

0.0 0.5

1 o 1.; 2.0

2.5 30

0, (i 012 m”)

FIG. 3. AE* from PL vs total n, from SdH on samples with one and two

occupied subbands. The solid and dashed lines indicate the 2DDOS

curves derived from CR. The inset shows typical CR spectra at T = 2.2 K

with different magnetic fields.

curves are in good agreement with n, values from SdH

within an accuracy of about 10%.

In conclusion, the potential of PL for a contactless and

rapid determination of the carrier density in PM HFET

structures has been demonstrated for IZ, n the range from

1.1 to 2.6X 1012cm-‘. N, values are obtained from the PL

spectral width and the 2DDOS reference curve. The values

are in excellent agreement with SdH results. The EF cutoff

appears clearly for kT<EF which makes this technique use-

ful for the determination of n, at low temperature. To in-

vestigate the potential of PL to give reliable n, values at 300

K, further experiments and a spectrum line shape analysis

are under way.

We would like to thank F. J. Berlec and R. Trapp for

technical assistance and H. J. Herzog for x-ray data. One

of us (H.M.) would also like to thank R. Sauer for useful

discussions. This work was partly supported by the

Bundesministerium fur Forschung -, und Technologie

(Bonn) under Contract No. NT 2754.2.

’ Modulation-Doped Field-Effect Transistors, Applications and Circuits;

edited by H. Dlimbkes (IEEE, New York, 1991).

‘L. D. Nguyen, D. C. Radulescu, M. C. Foisy, P. J. Tasker, and L. F.

Eastman, IEEE Trans. Electron Devices 36, 833 (1989).

the oscillation period the subband popu-

ple No. 9 indicating the occupation of two

is accurately determined via ni = (4e/h)[l/

( l/B,)i]. The values are in excellent agreement with n,

alues from PL (see Table I). For the determination of n,

PL via Eq. ( 1) we use the 2DDOS, which is m*/di2

subband. We performed far-infrared transmission

to get information about m*. Pronounced dips

the transmission indicate the excitation of cvclotron res-

and J. M. Ballingall, IEEE Electron Device Lett. 10, 580 ( 1989).

4B. D. Joyce and E. W. Williams, Proceedings of the International Sym-

‘M. Y. Kao, P. M. Smith, P. Ho, P. C. Chao, K. H. Duh, A. A. Jabra,

posium on GaAs and Related Compounds, Aachen, 1970 (Institute of

Physics, Bristol, 1971), pp. 57-63.

5T. P. Pearsall, .L. Eaves,- and J. C. Portal, J. Appl. Phys. 54, 1037

(1983).

‘I. V. Kukushkin, K. v. Klitzing, and K. Ploog, Phys. Rev. B 37, 8509

(1988), and references therein.

‘Y. H. Zhang, D. S. Jiang, R. Cingolani, and K. Ploog, Appl. Phys. Lett.

56,- 2 195 ( 1990), and references therein.

.

inset of Fig. 3). From the CR-position w, the

mass at EF is determined via

Extrapolated to B= 0 we found

= 0.07 Imo.

AE* values from PL and n, values from SdH are

. 3 together with the 2DDOS curves from Eq.

( 1) using m* from CR. The energetic onsets of the popu-

of the second subband are taken from the @ PL

and are marked by arrows. This leads to three

curves for samples Nos. 4, 5, and 9 due to differ-

ent x and Ii, IV, values deduced rom PL and 2DDQS

“W. Chen, M. Fritze, A. V. Nurmikko, D. Ackley, C Colvard, and H.

Lee. Phys. Rev. Lett. 20, 2434 ( 1990).

“M. S. Skolnick, K. J. Whittaker, P. E. Simmonds, T. A. Fisher, M. K.

Saker, J. M. Rorison, R. S. Smith, P. B. Kirby, and C. R. H. White,

Phys. Rev. B 43, 7354 (1991), and references therein.

‘OH. Brugger, H. Miissig, C. Walk, F. J. Berlec, R. Sauer, K. Kern, and

D. Heitmann, Proceeding of the International Symposium on GaAs and

Related Compounds, Seattle, 1991 (Inst. Phys. Conf. Ser., Bristol,

1992), (in press).

“C. Colvard, N. Nouri, H. Lee, and D. Ackley, Phys. Rev. B 39, 8033

(1989).

“M S Skolnick, K. J. Nash. M. K. Saker, S. J. Bass, P. A. Claxton, and.

J. S. Roberts, Appl. Phys. Lett. 50, 1885 (1987).

13S. K. Lyo and E.

D. Jones,

Phys.Rev.~B 8, 4113 (1988).

2741 Appl. Phys. Lett., Vol. 59, No. 21, 18 November 1991

Brugger et al.

2741

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