Growth and Characterization of Piezoelectrically Enhanced Acceptor-Type AlGaN/GaN Heterostructures
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This paper examines the extent of piezoelectric acceptor doping and modulation doping and compares these results with conventional Mg acceptor doping in AlGaN/GaN system. Material characterization including the Mg concentration profile and AlGaN/GaN interface, as determined by secondary electron mass spectrometry (SIMS), are discussed in relation to the sheet conductivity of the films. EXPERIMENTAL PROCEDURE Each heterostructure consisted of a GaN (0001) film grown at 1000ºC on an AlxGa1-xN (0001) layer. The latter was deposited at 1020ºC on an AlN (0001) buffer layer previously deposited at 1100ºC on an on-axis 6H-SiC (0001) substrate. Figure 1 shows the doping and Al profiles of the GaN and AlxGa1-xN layers in selected samples. xAl 20% 10% [Mg] 2E19
xAl 20% 10% [Mg] 2E19
800 1300 Depth (A) Sample A xAl 20% 10% [Mg] 2E19
xAl 20% 10% [Mg] 2E19 800 1300 Depth (A) Sample B
No AlGaN
800 1300 Depth (A) Sample C
xAl 20% 10% [Mg] 2E19 75 800 1300 Depth (A) Sample D
75 800 1300 Depth (A) Sample E
Figure 1. Doping and Al profiles in selected GaN/AlxGa1-xN heterostructures. The zero point of the depth scale is the top of the GaN layer. The AlN buffer layer and all subsequent films were grown in a cold-wall, vertical, pancake-style, RF inductively heated metalorganic vapor phase epitaxy (MOVPE) system. Ammonia (NH3), triethylaluminum (TEA) and triethylgallium (TEG) were used as precursors. Bis-cyclopentadienyl-magnesium (Cp2Mg) was employed for the p-type doping. High-purity H2 was used as both the carrier and the diluent gas. After cooling to room temperature the samples were annealed at 800°C in N2 to activate the Mg acceptors. Additional details of the growth experiments in the NCSU reactor have been previously reported [8]. Sample A contains all doping contributions: piezoelectric (PZ), modulation doping (MD) and conventional acceptors (ACC). Sample B lacks the modulation doping component. Sample C is an Mg-doped GaN film and is the control sample. It has no PZ charge or modulation doping. Sample D has doping from MD and PZ. Sample E has only PZ doping. The Mg concentration profiles and the Al concentrations were determined using Secondary Ion Mass Spectrometry (Cameca IMS-6f) having a 100nA 10keV O 2+ primary beam. Sputtering rates and Mg sensitivity factors for varying Al concentrations have been previously determined [9]. Analysis of the AlGaN/GaN interface was conducted by comparing the Al and Ga signals. Electrical measurements of the films
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were conducted using CV and Hall measurements (van der Pauw configuration). The Au(400A)/Ni(600A) ohmic contacts were produced by evaporization of the individual metals and subsequently alloying at 650ºC for 20 minutes. The sheet resistance and the Hall mobility were determined for each layer. RESULTS AND DISCUSSION Magnesium depth profiles determined in Samples A-E are shown in Figure 2.
Mg Concentration (atoms/cm3)
1.00E+21 Sample A Sample B Sample C Sample D Sample E
1.00E+20
1.00E+19
1.00E+18
1.00E+17
1.00E+16
1.00E+15 0
1000
2000
3000
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