Monitoring and Controlling of Strain During MOCVD of AlGaN for UV Optoelectronics
- PDF / 325,837 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 79 Downloads / 164 Views
responding author. Present address: Sandia National Laboratories, MS-0601, P.O. Box 5800, Albuquerque, New Mexico 87185-0601, Fax: 505-844-3211, email: [email protected]
Downloaded from https://www.cambridge.org/core. IP address: 5.189.207.244, on 14 Mar 2019 at 12:49:09, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S109257830000346X
observed during fabrication of AlGaN/GaN UV LEDs with thick AlGaN barriers (Figure 2a). The presence of cracking causes a significant variation of current-voltage characteristics among the tested devices and contributes to a large leakage current under reverse-bias conditions (Figure 2b). It is worth noting that cracking of AlGaN layers on thick GaN has been reported previously [6, 7]. case (b)
case (a)
p-GaN
p-GaN
p-AlGaN
p-AlGaN p-AlGaN n-AlGaN
p-GaN n-GaN
n-AlGaN n-InGaN
n-AlGaN
n-GaN
n-GaN
(c) tension
(a)
compression Strain*thickness (A. U.)
(b)
Figure 1. Schematic diagrams of a blue laser diode (a) and a UV LED (b). The indium-containing layers are labeled in blue and the AlGaN layers are colored in red. (c) Strain-thickness product along the growth direction for the structures of (a) (dashed line) and (b) (solid yellow line). InGaN layers tend to move the curve toward blue (compression) and AlGaN layers to red (tension).
ND0303B 100um devices 0.01
Current(mA)
(b) 0.005 0 -0.005
(a) -0.01
-4
-2
0 2 Voltage(V)
4
Figure 2. (a) Top view of an etched circular mesa (100 µm diameter) showing the presence of a high density of cracks. (b) Diode I-V curves taken from various devices across the same cracked sample.
An additional complication arises for AlGaN grown on sapphire, the most common substrate of choice, as the sapphire (linear thermal expansion coefficient α ~ 7.6x10-6 k-1) exerts a compressive strain to the AlGaN layers (α ~ 5.6x10-6 k-1) during cool down which tends to mask the grown-in tensile strain due to lattice mismatch. Most of the post-growth ex-situ strain characterizations [8-13] would in this case measure a combination of a tensile stress due to lattice mismatch and a compressive component due to thermal expansion mismatch. In an attempt to
Downloaded from https://www.cambridge.org/core. IP address: 5.189.207.244, on 14 Mar 2019 at 12:49:09, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S109257830000346X
isolate these two competing factors by directly probing the grown-in strain, we have employed an in-situ stress/strain monitor based on wafer-curvature measurement [14]. In this paper we will report the monitoring and subsequent control of grown-in strain of AlGaN on sapphire using different buffer layer schemes. EXPERIMENT A high-speed (1200 rpm), inductively heated, rotating disk reactor (RDR) was used to deposit GaN films (nominally 1-3 µm thick) onto 2” diameter, 330µm thick, (0001) sapphire wafers. Trimethylgallium, trimethylaluminum, and ammonia where used as the precursors, with hydrogen as
Data Loading...
