The investigation of stress in freestanding GaN crystals grown from Si substrates by HVPE

Abstract

We investigate the stress evolution of 400 µm-thick freestanding GaN crystals grown from Si substrates by hydride vapour phase epitaxy (HVPE) and the in situ removal of Si substrates. The stress generated in growing GaN can be tuned by varying the thickness of the MOCVD AlGaN/AlN buffer layers. Micro Raman analysis shows the presence of slight tensile stress in the freestanding GaN crystals and no stress accumulation in HVPE GaN layers during the growth. Additionally, it is demonstrated that the residual tensile stress in HVPE GaN is caused only by elastic stress arising from the crystal quality difference between Ga- and N-face GaN. TEM analysis revealed that the dislocations in freestanding GaN crystals have high inclination angles that are attributed to the stress relaxation of the crystals. We believe that the understanding and characterization on the structural properties of the freestanding GaN crystals will help us to use these crystals for high-performance opto-electronic devices.

The investigation of in situ removal of Si substrates for freestanding GaN crystals by HVPE

“…Previously, we discussed the optimal temperature to etch Si substrate in order to avoid the undesirable tensile stress between the substrate and GaN. 19,20 We found that the maximum in-plane stress can be minimized to À0.07 MPa at 1000 C being slightly compressive and allowing the implementation of Si etching. However, the uniformity of Si etching rate over 2-inch wafer surface is another important factor to obtain crack-free freestanding GaN.…”

“…The growth process is reported elsewhere in details. 19,20 Briey, GaN was grown at 1080 C in atmosphere pressure with V/III ratio of 20. The Si etching was performed using 200 sccm HCl gas ow at 1000 C aer the growth of a thick GaN layer.…”

“…Recently, we reported on the high quality freestanding GaN with an etch pit density (EPD) of about 1 Â 10 6 cm À2 over the 2-inch wafer grown using a Si substrate by chemical etching of substrate material aer GaN growth. 19,20 Even if the EPD of the freestanding GaN crystal obtained using in situ removal of a Si substrate fall short of EPD for homoepitaxially-grown material ($10 4 cm À2 ), its value is comparable to commercial HVPE freestanding GaN grown on Al 2 O 3 substrates (TDD < 1 Â 10 7 cm À2 ). 21 This technique will allow us to obtain freestanding GaN wafer with a large scale and low fabrication cost, thus enabling to lead to their successful commercialization.…”

First observation of electronic trap levels in freestanding GaN crystals extracted from Si substrates by hydride vapour phase epitaxy

“…Moreover, they addressed that these strain-induced metallic bond states can be observed in the GaN crystals grown by MOCVD or HVPE, which contain the abundant stress and nitrogen concentration in the layer 39 . It is noticeable that Si-based FS-GaN underwent a huge tensile strain during the HVPE growth 16 . Furthermore, we also observed metallic Ga-Ga bonding in GaN template on a Si substrate, which underwent high tensile strain during the growth stage 40 .…”

“…The FS-GaN crystals used in this study were grown from Si substrates, using an in-situ removal method by HVPE. The details of the growth process are described elsewhere 15,16 . A home-built vertical type HVPE system with an upstream HCl gas channel was used to achieve the FS-GaN crystal from a Si substrate.…”