Photoreflectance investigations of HEMT structures grown by MBE

ARTICLE IN PRESS

Journal of Crystal Growth 278 (2005) 591–595 www.elsevier.com/locate/jcrysgro

Photoreflectance investigations of HEMT structures grown by MBE L. Zamora-Peredoa, M. Lo´pez-Lo´pezb, A. Lastras-Martı´ neza, V.H. Me´ndez-Garcı´ aa, a

Optical Communications Research Institute (IICO) and Facultad de Ingenierı´a, Universidad Auto´noma de San Luis Potosı´, Av Karakorum 1470, Lomas 4a Seccio´n, 78210 San Luis Potosı´, S.L.P., Me´xico b Physics Department, Centro de Investigacio´n y de Estudios Avanzados del IPN, Apartado Postal 14-740, 07000 Me´xico, D. F., Me´xico Available online 30 January 2005

Abstract High electron mobility transistor (HEMT) structures consisting of: GaAs (buffer layer)/i-AlGaAs(spacer layer)/nAlGaAs(barrier layer)/n-GaAs(cap layer) were grown by MBE on GaAs substrates. The optical properties of the HEMT structures were studied by photoreflectance (PR) spectroscopy at different temperatures. We observed in the PR spectra transitions located approximately at 1.41 and 1.85 eV, associated with the energy band edges of GaAs and AlGaAs, respectively. Oscillatory signals slightly above these energies were recognized as Franz–Keldysh oscillations (FKO). By employing lasers with different wavelengths as modulation sources, and by varying the thickness of the layers in the heterostructures we were able to identify the origin of the PR signals in the spectra. The FKO close to the GaAs band gap originate at the GaAs buffer/i-AlGaAs heterojunction where the two-dimensional electron gas is localized. Finally, low-temperature PR experiments indicate that the oscillatory signal observed above the AlGaAs band edge is formed by two overlapped signals, one of which originates in the GaAs(cap)/n-AlGaAs interface. r 2005 Elsevier B.V. All rights reserved. PACS: 68.65.+g; 73.20.–r; 73.40.Hm; 73.40.Kp; 78.66.–w Keywords: A1. Interfaces; A1. Low-dimensional structures; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials; B3. High electron mobility transistor

1. Introduction

Corresponding author. Tel.: +52 444 8 25 01 83; +52 444 8 25 01 98. E-mail address: [email protected] (V.H. Me´ndez-Garcı´ a).

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Photoreflectance (PR) spectroscopy is now widely used for the study and characterization of semiconductor device structures, like heterojunction bipolar transistors (HBT) [1,2], high electron mobility transistors (HEMT) [3,5–14], and vertical

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cavity semiconductor emitting lasers (VCSEL) [4]. This modulation technique is a powerful tool to investigate distinctive properties of low-dimensional semiconductor heterostructures [5]. The PR technique has been extensively used for the study of HEMT structures, with either GaAs [5–14] or InGaAs [3] containing the high mobility channel of the device, in order to reveal in an easy and non-destructive mode different parameters of the samples like the energy band edges, Al (or In) concentrations and internal electric fields. Actually, three types of signals are observed in the PR spectra of HEMT structures, the first signal contains Franz–Keldysh oscillations (FKO) close to the GaAs gap energy region, the second is a broad oscillation (BO) extending over a wide spectral range (from 1.43 to 1.78 eV), and the third signal is FKO associated with the E 0 transition of the AlGaAs. While there seems to be a general consent that BO originates in the cap layer [6–9,12,14], and the FKO-AlGaAs in the spacer layer region, there is no consensus about the origin of the first signal. Soares et al. [9] and Novellino et al. [10] associated these oscillations to the built-in electric field localized on the substrate–buffer layer interface, while Glembocki et al. [11], Sydor et al. [12] and Sek et al. [13] related these oscillations to the 2DEG. In this work, we investigated the origin of FKO above the band gap energies of GaAs and AlGaAs observed in the PR spectra of HEMT structures. Varying the thickness of the layers in the HEMT structures and using three different wavelength lasers as modulation sources, we changed the modulation depth to separate the signal from different interfaces.

2. Experimental procedure The HEMT structures were grown by molecular beam epitaxy (MBE), on semi-insulating GaAs substrates. A 3 mm undoped GaAs buffer layer (BL) was deposited, followed by an undoped Al0:35 Ga0:65 As spacer layer with different thickness S ¼ 60, 120 and 180 A˚ for the samples M1, M2 and M4, respectively. Next, a 1000-A˚-thick Al0:35 Ga0:65 As barrier layer doped with Silicon at

λ = 632.8 nm λ = 543.5 nm λ = 325 nm GaAs capping

AlGaAs Barrier

100

Σ

GaAs Substrate Buffer Layer

1000 10000 Thickness (Å)

100000

Fig. 1. Layer diagram of the HEMT structures studied in this work. The arrows illustrate the penetration depth of the three lasers employed in the PR measurements.

1.3  1018 cm3 was grown, and finally the structures were capped with 100 A˚ GaAs films with the same Si-doping concentration. A schematic representation of the HEMT structure is shown in Fig. 1. Other sample (M3) with 150-A˚-thick spacer layer and 500 A˚ n-AlGaAs barrier was grown. The PR measurements were performed by means of a standard experimental setup [5]. The probe beam was generated by the monochromatized light of an 80 W tungsten halogen lamp, focused on the sample. Three different lasers were used as modulation sources alternatively: HeNe (632.8 and 543.5 nm) and HeCd (325 nm). A cold finger setup was employed to perform lowtemperature PR measurements.

3. Results and discussion Fig. 2 shows typical room temperature PR spectra from three HEMT structures obtained with the 543.5 nm wavelength laser. The line-shape observed is very similar to those reported for similar structures [5–13]. In the spectra several signals can be identified as marked in Fig. 2: (I) the GaAs band edge (BE) at 1.42 eV; (II) FKO associated with the GaAs-BE (FKOGaAs ); (III) BO region from 1.43 to 1.78 eV associated with the surface electric field [9], and electron–hole transitions in the surface quantum well localized in the cap layer [14]; (IV) the AlGaAs BE transition; and (V) the FKO associated with the AlGaAs-BE (FKOAlGaAs ).

ARTICLE IN PRESS L. Zamora-Peredo et al. / Journal of Crystal Growth 278 (2005) 591–595

I

II

III

IV

V

∆R/R (a.u.)

M1

M2

M4

1.4

1.5

1.6

1.7 1.8 1.9 Energy (eV)

2.0

2.1

2.2

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modulation range close to 1000 A˚ [12]. Therefore, signals from the topmost layers are expected to be dominant in the PR spectra. Fig. 3 shows the PR spectra obtained with the HeCd laser for the same HEMT structures analyzed in Fig. 2. We observed BO and AlGaAs signals similar to those depicted in Fig. 2, but FKOGaAs almost disappears. This result is explained considering that the sum thickness of the topmost layers (spacer+barrier+cap layer), which is approximately 1200 A˚, is slightly larger than the modulation depth to reach the BL–AlGaAs interface. Therefore it is thought that the origin of FKOGaAs lies in the BL–AlGaAs interface where the 2DEG is located. The BL–substrate interface is discarded as possible origin of FKOGaAs ; since it is about 3 mm away from the modulated region. Fig. 4 shows PR spectra from sample M3 obtained with modulation of different lasers: 632.5, 543.5 and 325 nm. As described previously, the thickness of the n-AlGaAs barrier (500 A˚) in

Fig. 2. Room temperature PR spectra of M1, M2 and M4 samples using the 543 nm line of a HeNe laser as modulation source. In the spectra five regions can be identified as marked by I–V in the figure.

M1

∆R/R (a.u.)

From an analysis of the FKO we can determine the internal electric fields strength, as well as the energy gap by means of the well-known method developed by Aspnes and Studna [15]. The built-in electric field magnitude obtained from the FKOGaAs is around 6  105 V/m, while that obtained from the FKOAlGaAs is around 3  107 V/m, indicative of a different region of origin. Concerning the FKOGaAs ; it is not clear if they come from the substrate–BL interface or the BL–AlGaAs heterojunction. In the PR spectra of Fig. 2, the penetration depth for the 543.5 nm wavelength is estimated to be 200 nm and the indirect modulation depth is larger than 3 mm, making it difficult to establish the responsible interface of FKOGaAs ; because all interfaces are modulated with this laser. In order to split the signals associated with each interface, we used a 325 nm wavelength HeCd laser with a shorter penetration depth in the HEMT structure, estimated to be 13 nm and with a

M2

M4

1.4

1.5

1.6

1.7 1.8 1.9 Energy (eV)

2.0

2.1

2.2

Fig. 3. Room temperature PR spectra of M1, M2 and M4 samples using the 325 nm line on a HeCd laser as modulation source. FKOGaAs almost disappears because of the shorter penetration depth of this laser.

ARTICLE IN PRESS L. Zamora-Peredo et al. / Journal of Crystal Growth 278 (2005) 591–595

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∆R/R (a.u.)

λ = 632.8 nm

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Σ = 60Å

M2

Σ = 120Å

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Σ = 180Å

M4

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λ = 543.5 nm 1.48

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1.4

1.5

1.6

1.7

1.8

Excitation-line 543.5 nm 325.0 nm

77 K

1.9

Energy (eV)

18 K

Fig. 4. PR spectra of M3 taken with three different modulation sources. The inset shows PR spectra of M1, M2 and M4 close to the GaAs band gap energy region. The FKO (GaAs) does not disappear because the AlGaAs layer thickness is thinner than the modulation deep.

this sample is half of that utilized in samples M1, M2 and M4. Note that FKOGaAs do not disappear even after using the 325 nm wavelength as the modulation source. The internal electric field magnitude for M3 determined through FKOGaAs was 4  105 V/m for the three modulation wavelengths. Therefore, PR measurements in Fig. 4 confirm that the FKOGaAs originate in the BL–AlGaAs spacer layer heterojunction, since for sample M3 this region is surely modulated with the 325 nm laser line. Furthermore, in the inset of Fig. 4 is shown that FKOGaAs invert their phase when the spacer layer thickness is varied, i.e. when the region of the 2DEG is modified. This effect could be associated with the optical delay caused by the change of thickness. Finally, PR measurements at 300, 210, 77 and 18 K were made in order to investigate the origin of the FKO observed above the AlGaAs band gap, FKOAlGaAs : Fig. 5 shows the PR spectra obtained for M4 using the 543.5 nm (thick line) and the 325 nm (thin line) wavelength laser at each temperature. In this figure, we observe that the FKO(GaAs) disappear at the laser wavelength of 325 nm. Note that the 2DEG region for sample

1.4

1.6

1.8 2.0 Energy (eV)

2.2

2.4

Fig. 5. PR spectra of M4 at different temperatures using 543.5 nm (thick line) and 325 nm (thin line) wavelength lasers as modulation sources. Note the disappearing of FKO (GaAs) at the wavelength of 325 nm with a shorter penetration depth.

M4 is the deepest of the studied samples. Therefore the short penetration depth of the 325 nm laser could leave the 2DEG region without modulation in this sample. We observe that the FKOAlGaAs are well defined in the spectra taken at room temperature. By analyzing the FKOAlGaAs we were able to determine the electrical field strength, 3  107 V/m, as well as the Al concentration, 35%, these values are in good agreement with the nominal growth parameters. As the temperature is decreased, the FKOAlGaAs oscillations are distorted and the FKO analysis to calculate the electric fields is no longer possible. Two overlapping signals are suggested at low temperatures. As previously explained the differences in the PR spectra obtained with lasers of different wavelength must be related to the change

ARTICLE IN PRESS L. Zamora-Peredo et al. / Journal of Crystal Growth 278 (2005) 591–595

in modulation depth. However, note that in Fig. 5 the PR spectra for both modulation lasers are quite similar. Sydor et al. [16] selectively modulated interfaces in HEMT heterostructures by using differential photoreflectance (DPR). They found that if the cap/AlGaAs interface is not modulated, the entire AlGaAs signal (along with FKOAlGaAs ) is eliminated. Therefore, our results confirm the observations reported in Ref. [16], since both lines are able to modulate the cap/ AlGaAs region where the origin of FKO(AlGaAs) has been associated.

4. Conclusions We have investigated the PR signals associated with buffer–AlGaAs and AlGaAs–cap layer interfaces of HEMT structures. PR measurements were performed with different wavelengths as modulation sources and at different temperatures. We found that the FKO close to the GaAs band gap originate from the electric field in the 2DEG region, then we can estimate the magnitude of the electric field in these interfaces. Besides, we found that the FKO above the AlGaAs band gap actually are originated on the AlGaAs-cap layer.

Acknowledgments This work was partially supported by CONACyT-Mexico, FAI-UASLP, and PROMEP. The authors would like to express their gratitude to

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