Bor nitrür üretimi

TÜBİTAK MAG15.04.2008Bor nitrür, grafite çok benzer altıgen (h-BN) yapıda tabakalar halinde veya kübik yapıda (k- BN) elmasa çok yakın özelliklerde bulunabilir. k-BN bilinen malzemeler içinde elmastan sonra en sert olduğundan malzeme endüstrisinde sert metal kaplamalar yapmada (elmastan daha üstün özelliklerde, metal işlemede) kullanılmaktadır. Ayrıca, elmas sadece p-türü katkılanabildiği halde, k-BN hem p hem de n türü katkılanabilmektedir, ve dolayısıyla elektronik devrelerin yapı taşı olan p-n eklemini üretmek olası olduğundan mor-mavi ışık bölgesinde ışık algılayıcısı (detektör) ve yayınlayıcısı (LED) uygulamasına açıktır. k-BN tabanlı bu devre elemanlarının, opto-elektronik yatkınlıkları yanında, elmastan daha yüksek bir yasak enerji aralığına sahip olmalarından dolayı çok daha yüksek sıcaklık ortamlarında kullanılmalarını sağlanabilir. Son yıllarda yapılan araştırmalar k-BN’nin plazma ortamında Fiziksel Buhar Biriktirme (FBB) veya Kimyasal Buhar Biriktirme (KBB) yöntemleri ile üretilebileceğini göstermiştir. Ancak bu çalışmalar biriktirilen maddenin özelliklerine (k-BN içeriği ve mekanik gerilim) ve birikme hızına etki eden üretim parametrelerinin (kullanılan bor ve azot kaynakları, kullanılan gaz kompozisyonu, kaplanan yüzeyi oluşturan madde(taban), uygulanan plazma yoğunluğu, RF gücü, bias voltaj, taban sıcaklığı) nasıl etki ettiğini sistematik bir yaklaşımla incelememiştir. Elde, çevre birimleriyle birlikte, kurulu bulunan hem FBB, hem de KBB düzenekleriyle (yapılabilir bazı değişiklikler ve eklerle), yukarıda sözü edilen sistematik çalışma olanaklar çerçevesinde gerçekleştirilmiştir. Başka bir deyişle hem RF hem de MW kullanan KBB ve magnetron çığlama kullanan KBB teknikleriyle büyütülen filmler, eldeki ve proje bütçesiyle sağlanan olan ölçüm/test düzenekleriyle çözümlenmiştir. Böylece üretim test döngüsü yinelenerek hedeflenen mekanik ve opto-elektronik özellikte k-BN ince filmleri ve ondan üretilebilecek yapıları oluşturan en uygun üretim koşulları belirlenmeye çalışılmıştır.Boron nitride can be found in hexagonal structure (hBN) which is very much like graphite or in cubic structure with properties very close to those of diamond. Since cBN is the hardest known material after diamond is used in making hard metal covers (used in metal machining with superior properties to diamond). In addition, while diamond can be doped only in p type both p and n type doping is possible in cBN, therefore cBN can be used to make p-n junction which is a basic part of the microelectronic circuits. That means cBN can be used to make a detector or Light Emitting Diode (LED) in violet-blue region. In addition to these optoelectronic properties, cBN based circuit parts are expected to withstand very high temperatures due to the higher forbidden energy gap of cBN compared to that of diamond. Recent studies have shown that cBN can be produced by Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) in plasma. But these studies have failed to determine) how all of the production parameters (boron and nitrogen sources, composition of the gas used, the material covered (substrate), plasma density, RF power, bias voltage, substrate temperature) affect on the properties (cBN content and mechanical stresses) and the deposition rate of the product with a systematic approach. The systematic study was realized in the range of available experimental ability of the present PVD and CVD equipment and accessories with some possible additions and changes. The cBN films were produced in the plasma equipment and was studied with the measurement and testing facilities that already exist in addition to measurement and testing equipment acquired within the budget of this project. The optimum production conditions of cBN with desired mechanical and optoelectronic properties were studied experimentally

Stability and degradation of plasma deposited boron nitride thin films in ambient atmosphere

Boron nitride (BN) thin films were deposited at 296 K, 398 K, 523 K and 623 K by low power radio frequency plasma enhanced chemical vapor deposition with nitrogen (N-2.) and hydrogen diluted diborane (15% B2H6 in H-2) source gases. Fourier transform infrared and UV-visible spectroscopies were used to investigate the stability and degradation of BN films under ambient air conditions. The action of moisture on the films is reduced with increasing substrate temperature (T-s) to the detriment of the film growth rate. This has been interpreted as related to the decrease in porosity and relative volume fraction of B-O containing disordered tissue at higher T-s The thickness of the unstable films increases logarithmically with the air exposure time. Parallel to this, although the E-o4 gap increases logarithmically with time, the Tauc gap remains the same. The increase of subgap absorptions and the decrease of Tauc slope with time indicate reduction of structural order. Crystallites of ammonium borate hydrates, the main product of the chemical reactions, are initially formed within the bulk. At a later time, as a result of increased porosity and disorder, the film thickness decreases while the islands of micro-crystallites rapidly grow above the surface of the film. Stability dependence on other deposition parameters was also studied: it is found that the 1260/1360 cm(-1) (O-B-O/B-N) infrared peak area ratio plays an indicator role to reveal the stability of BN films

Large area uniformity of plasma grown hydrogenated nanocrystalline silicon and its application in TFTs

This work investigates the effect of RF power density (100-444 mW/cm(2)) on the structural, optical and electrical properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma enhanced chemical vapor deposition (PECVD) technique. Moreover, RF power effect on the large area uniformity was analyzed by comparing the properties of the films deposited at the center and near the edge of the PECVD electrode. Film characterization was performed by Fourier transform infrared and UV-visible spectroscopies, X-ray diffraction and current-voltage measurements. The films at the center of the electrode show variations in their properties with RF power, while the ones near the edge exhibit almost no dependence on RF power. With increasing RF power, the film structure at the center of the electrode transforms from amorphous to nanocrystalline, the dark resistivity (rho) decreases from similar to 1 x 10(8) Omega cm to similar to 1 x 10(5) Omega cm, while the Urbach tail (E(0)) expands from similar to 50 meV up to similar to 200 meV. For all powers, the films near the edge of the electrode, apart from their nanocrystalline structure, have higher crystalline volume fraction, microstructure factor and E(0), and lower rho and hydrogen content (C(H)) compared to the films at the center. The influences of the power density on the uniformity of the film properties are discussed in the frame of nc-Si:H film growth models. At the highest available RF power density (444 mW/cm(2)), nc-Si:H films show a slight deterioration in their properties and uniformity, revealing an optimum power density of similar to 300 mW/cm(2), where the lowest rho and C(H) together with high uniformity is reached. Bottom-gate thin film transistor (TFT) with the nc-Si:H channel layer deposited at this optimum power exhibits lower threshold voltage, better electrical stability and slightly higher mobility compared to the TFT with the channel grown at 100 mW/cm(2)

Photoluminescence analyses of hydrogenated amorphous silicon nitride thin films

Silicon-rich hydrogenated amorphous silicon nitride (a-SiN(x):H) films were grown by plasma enhanced chemical vapor deposition (PECVD) with different r=NH(3)/SiH(4) gas flow ratios. The optical absorption characteristics were analyzed by Fourier transform infrared (FTIR) and UV-visible transmittance spectroscopies. The recombination properties were investigated via photoluminescence (PL) measurements. As r was increased from 2 to 9, the PL emission color could be adjusted from red to blue with the emission intensity high enough to be perceived by naked eye at room temperature. The behaviors of the PL peak energy and the PL band broadness with respect to the optical constants were discussed in the frame of electron-phonon coupling and band tail recombination models. A semiquantitative analysis supported the band tail recombination model, where the recombination was found to be favored when the carriers thermalize to an energy level at which the band tail density of states (DOS) reduces to some fraction of the relevant band edge DOS. For the PL efficiency comparison of the samples with different nitrogen contents, the PL intensity was corrected for the absorbed intensity fraction of the incident PL excitation source. The resulted correlation between the PL efficiency and the subgap absorption tail width further supported the band tail recombination model

Annealing improvement on the localized states of plasma grown boron nitride film assessed through admittance measurements

Boron nitride (BN) thin film was grown by plasma enhanced chemical vapor deposition (PECVD) technique and was investigated by UV-Visible transmission, Fourier transform infrared (FTIR) and ac conductance spectroscopies. Mainly the density of electronic localized states (D-it) at BN/Si interface was obtained by continuum and statistical models of ac conductance through an MIS structure (Al/BN film/Si). The origins of the electronic defects have been outlined and discussed within the frame of a nitrogen deficient turbostratic structure where more or less parallel hexagonal crystallites of distributed size would be embedded in a disordered phase. The nitrogen deficiency of the film was tried to be restored by annealing treatment under nitrogen atmosphere at two temperatures

Instability phenomenon originated from the disordered layer of the plasma-deposited BN film/c-Si interface assessed through the MIS structure by admittance measurement

A plasma enhanced chemical vapor-deposited (PECVD) boron nitride (BN) film on p-type crystalline silicon (p-c-Si) was used to fabricate the metal-insulator-semiconductor (MIS) test structure. The effects of positive (inverting type) and negative (accumulating type) bias stresses on the MIS (Al/BN/p-Si/Al) structure were investigated as a function of time, bias voltage stresses and temperature. Charge injection into the gate dielectric (BN film in this case) was considered as a mechanism to provoke the instability problem that manifested itself as a slow shift of specific voltage (Delta V-HH) which symbolizes the whole capacitance-gate bias voltage (C-V) curve. Moreover, the evolution of this shift is followed by monitoring Delta V-HH as a function of time, temperature and gate voltage stress and fitted to a functional form for the Delta V-HH shift kinetics. Good agreement with the experimental data confirms the charge injection hypothesis behind the shift in C-V curves. Finally, the origin of an eventual defective structure, required by the actual instability, is argued in terms of possible chemical composition of the film. Seemingly BN possesses a turbostratic structure; the first layer at the initial stage in growth is amorphous (even self-doped by a contamination from the underlying silicon substrate) on which a more ordered phase might continue

Electrical transport mechanism in boron nitride thin film

Both dc and ac transport characteristics of plasma enhanced chemical vapor deposited (PECVD) boron nitride (BN) thin film was investigated by dc current vs. dc voltage measurement at different temperatures and admittance vs. gate voltage at various frequencies/temperatures, respectively. MIM metal (Al)-insulator (BN)-metal (Al) or MIS metal (Al)-BN-semiconductor (p-Silicon) test devices were conventionally produced. Both conductivity anisotropy and dc/ac detailed transport mechanism were analyzed within the frame of a turbostratic structure (t-BN), interfacing the substrate by a thin amorphous layer (a-BN). This defective BN film has been justified by both infrared (IR) analysis and indirectly by the resulting electrical transport behavior. Transport and its Variations as a function of temperature/frequency are in agreement with a hopping mechanism across Gauss-like energy distributed localized deep traps