ICMBE 2008, Aug. 3~8th, 2008 at University of British Columbia, BC, Canada

This year’s MBE conference focused on the following main topics:

1. Self-assembled nanostructures: Quantum Dots, Quantum Wires on compound semiconductors and Si/Ge material systems.
2. Nitrides: Growth mechanisms, rf vs. NH₃ MBE, material qualities and a little bit of electronic and photonic device results.
3. Photonic devices: QD lasers, diluted nitride long wavelength lasers, quantum cascade lasers, LEDs and photodetectors
4. III-V MOSFETs
5. Widegap oxides
6. Solar cells
7. Large production scale MBE operations
8. Growth control: RHEED with rotation and temperature control

Some topics will be discussed below.

Multi-D Nanostrucutres:

Multi-dimensional nanostructures and devices continue to attract researchers’ interests. The studies range from III-V quantum dot (QD) lasers for telecommunication applications (QD lasers were finally commercialized after being proposed ¼ century ago); to Si/Ge island formation for Si hole mobility enhancement in CMOS applications; to vertical nanowire (whisker) growths on various of semiconductor substrates for practical applications yet to be realized. All these show that crystal growers are enthusiastic about finding a way to improve the lateral pattern control down to nanometer scale.

Nitrides:

There are several interesting papers on growing GaN heterostructures (using MBE of course). In the plenary talk PL2.2 Dr. Skierbiszewski (Institute of High Pressure Physics, Polish Academy of Science, Poland) described how high quality InGaN/GaN QW lasers can be grown using plasma-assisted MBE (PAMBE) with post-growth surface/optical/electrical properties surpass MOCVD-grown counterparts. By using extreme Ga-rich conditions close to the onset of Ga-droplet formation, a two-dimensional growth can happen at relatively low temperature (700~800^oC, compared to half of GaN melting point of 1050^oC, which is used by MOCVD growths). The Ga-rich condition produces a metallic Ga adlayer within which the nitrogen adatoms can diffuse with lowered diffusion barrier energies, and therefore grow two-dimensionally. What makes Dr. Skierbiszewski’s talk even more interesting is that in their institute they can grow GaN single crystal substrates with threading dislocation density <>-2, the synthesis happens in Ga solution at 1300-1600^oC under N₂ pressure of 8-17 kbar ( 1bar ~ 750 Torr ~ 1 atm, that’s why he’s from the Institute of High Pressure Physics). The size of the single crystal GaN is only about 1cm x 1cm but can be sold for 8~10 K USD a piece! I talked to him during the recess and discussed about whether they do GaN HEMT or not. I found out that somehow they rather grow green-blue lasers than GaN HEMTs (Blue/green lasers make more money than GaN HEMT ICs?). BTW, along the same direction I found out that Sumitomo also produces and sells single crystal 2” GaN substrates, but they need NDA if I want to discuss further with them.

NH₃ MBE for GaN attracts more and more attention, too. In my opinion (and maybe many other crystal growers’) energetic particles (in this case N₂^* radicals) coming out of rf-plasma source are not ideal nitrogen source. CNRS/Picogiga group presented several papers (WB1.6, THB2.1 invited, and THB2.2) on GaN layers grown by NH₃ MBE on Si(111). Nitrogen source is provided by injecting 100-200 sccm (as opposed to less than 10 sccm of N₂ for normal PAMBE with rf-plasma source) of NH₃ through a leak valve and thermally cracked on substrate. Therefore the growth temperature is ~ 100^oC higher than PAMBE. Besides the source of nitrogen, the development of III-nitride devices on Si substrate mostly relies on the management of the dislocation density and the residual strain. The CNRS/Picogiga group managed to grow high quality AlGaN/GaN HEMT structures with n_s ~ 10¹³ cm^-2 and μ_n ~ 2000 cm²/V-s (the number is comparable to, if not better than, MOCVD grown ones. As they said, mobility is not an issue for MBE-grown HEMT anymore). The tricks include first nitridizing the Si substrate surface by impinging NH₃ on cleaned hot Si(111) surface to form a very thin layer of single crystal Si₃N₄, this will produce a sharp bottom interface; then using an AlN/GaN/AlN 40nm/250nm/250nm stress mitigating layer to reduce/bend the dislocations resulted from the large lattice mismatch between Si(111) and AlN/GaN; finally grow a thick ( >1.5μm ) GaN buffer layer at an optimized temperature at which the dislocations incline and cancel each other (although the dislocation density of grown epi is still in ~ 10⁹ cm^-2 range). The AlGaN/GaN active layers are then grown on top of the buffer layer. They also monitored in-situ the curvature of the epi and established a relation between wafer bowing vs. epilayer dislocation density. The summary: convex bowing (means the GaN grows compressively strained on the stress mitigating layer) reduces the dislocation density. But there are more subtle interaction between dislocation density, leakage current of the HEMTs, and the donor density of the 2DEG. When dislocation density drops, the 2DEG donor density increases and the HEMT leakage current also increases.

Dislocation reduction and strain engineering are only two of many obstacles that need to be overcome to produce high quality and reliable GaN HEMTs, no matter what crystal growth technique chosen. Crack formation in grown epilayer resulted from thermal expansion coefficient mismatch between Si or SiC substrates and GaN (or even between AlN and GaN) is another issue, for example. Growing HEMTs on single crystal GaN substrates (if they get large enough and cheap enough) could theoretically solve most of the problems, but poor thermal conductivity of GaN (656 mW/cm^-1K^-1) compared to 6H-SiC (4900 mW/cm^-1K^-1) could potentially hamper its practical use.

MBE Large-Scale Production and Growth Control:

Besides nitrides there are also several papers on MBE growth control and large scale MBE production operation. Tom Rogers of RFMD shared their operation details (TUA2.1), which can be leveraged by us. Details include:

1. Minimal number of device structures
2. Several items can be used as production metrics:
1. System uptime, throughput, and yield
2. Manpower efficiency
3. Reproducibility inter MBE systems
4. Wafer uniformity and run-to-run reproducibility
3. Documentation, training and uniform methods are critical for multi-shift pass-downs
4. Using of band-edge thermometry (from kSA) to reduce temperature variation of wafers across a huge platen (7 x 6”, Veeco Gen2000) from 5.78^oC to 1.5^oC by optimizing the platen structure (wafer ledge and backing ring wideth), proper coating of platens with semiconductors, and fine-tuning the power ratio and distance between the heater and the platen (TUA2.3).
5. Precision MBE system downtime forecasting
6. 24 hr HBT QTA turn-around time
7. Pareto analysis of reject causes
8. SEMI-standard definitions and tool states

For growth control I found out that RHEED oscillation can be done on rotating substrates. It is simple in theory: synchronize the RHEED image capturing with substrate rotation. A rotation speed detection mechanism will be added by the CAR outside the vacuum and triggers the capture of RHEED intensity. In practice there are few concerns such as less available data points and therefore worse fitting; and the annoying wafer skidding. But it proves that RHEED works better than just monitoring substrate oxide desorption.