Patent Application
20160153119
Compared with sapphire and SiC substrates, Group-III nitrides epitaxy on Si substrate has many advantages: silicon based processing technologies are highly developed; high-quality, low-price and big-size Si substrates are available in the market; and Si substrates have good heat dispersion and can be easily removed after wafer-bonding. However, difficulties exist in Group-III nitrides epitaxy on Si substrates. For example, there are huge lattice mismatch and thermal mismatch between Si and Group-III nitrides, easily resulting in cracking of the epitaxial film; and Ga meltback etching may occur as Si reacts with Ga.
To solve the issue of film cracking, the Chinese Patent Application No. 201010137778.9 discloses method and structure of reducing LED epitaxial stress on Si substrates. According to that patent application, a silicon nitride or silicon dioxide layer is initially formed over the Si substrate surface through PECVD or sputtering, and afterwards a columnar or pitting circular structure is formed through photoetching. As it points out, in subsequent growth of Group-III nitrides, voids will be formed in the foresaid structure which help eliminate the tension between the film and the Si substrate. However, the processing technology disclosed in that patent application is complex, requiring additional equipment like PECVD and photoetching and thus incurring high processing cost.
The present disclosure provides an epitaxial structure of Group-III nitrides over the Si substrate and a forming method thereof. In this disclosure, Al atoms and in-situ formed SixNy coexists in the interface between Si and the epitaxial Group-III nitrides.
The interface structure characterized by “coexistence of Al atoms and in-situ formed SixNy” has the following advantages: 1) the provision of Al atoms which can be readily absorbed to Si is good for subsequent AlN epitaxy. In other words, Al atoms play the role of connecting the epitaxial Group-III nitrides and the Si substrate; 2) the SixNy produced in the interface between Si and the epitaxial Group-III nitrides releases the mismatch stress caused by heteroepitaxy; 3) this structure can be easily formed in-situ through MOCVD, MBE or HVPE, and integrated into the Group-III nitrides epitaxy.
According to a first aspect of the present disclosure, an epitaxial structure of Group-III nitrides is provided, comprising a Si substrate and a Group-III nitrides layer disposed over the Si substrate, wherein: Al atoms and in-situ formed SixNy are arranged in the interface between the Si substrate and the Group-III nitrides, wherein the Al atoms are configured to be absorbed to the Si substrate and to connect the Group-III nitrides layer and the SixNy is configured to release the mismatch stress caused by heteroepitaxy.
Further, some regions of the Si surface are covered by the Al atoms and some other regions of the Si surface are covered by the SixNy; and this structure of “coexistence of Al atoms and SixNy” is cladded by an AlN epitaxial layer.
Further, the thickness of the AlN epitaxial layer denoted by HAlN meets that 1 nm≦HAlN≦500 nm.
Further, the Group-III nitrides comprises a single-layer or multi-layer of following Group-III nitrides : AlN, AlxGa1-xN, GaN, InyGa1-yN or (AlxGa1-x)1-yInyN, where, 0<x<1, 0<y<1.
According to a second aspect of the present disclosure, a method for fabricating an epitaxial structure of Group-III nitrides is provided, which includes the following steps: (1) providing a Si substrate; (2) forming an interface structure over the Si surface, wherein the interface structure is arranged with Al atoms and in-situ formed SixNy, and both Al atoms and SixNy are cladded by an AlN epitaxial layer; and (3) further grow Group-III nitrides on the foresaid structure, in which, the Al atoms are configured to be absorbed to the Si substrate and to connect the Group-III nitrides, and the SixNy is configured to release the mismatch stress caused by heteroepitaxy.
In some embodiments, the interface structure of “coexistence of Al atoms and SixNy” can be formed in-situ through the following three epitaxial sub-steps: (i) introducing in an Al source for an appropriate period of time T1; (ii) closing the Al source and introducing in an N source for an appropriate period of time T2; and (iii) introducing in the Al source and the N source simultaneously to form an AlN epitaxial layer of certain thickness. Specifically, in sub-step (i), T1 is configured such that some regions of the Si surface are covered by the Al atoms and some other regions of the Si surface are not covered with the Al atoms due to the short exposure to the Al source; in sub-step (ii), T2 is configured such that SixNy is formed on the non-Al-covered regions of the Si surface, and that the Al-covered regions of the Si surface are protected from nitridation, and at the same time some of the Al atoms may be nitridated into AlN; , n; in sub-step (iii), T3 is configured such that the AlN epitaxial layer reaches a certain thickness in order to prevent the potential Ga meltback etching on the Si substrate.
An important aspect of this present disclosure is to determine the Al source exposure time T1 in step (1), so that the Si surface is not entirely covered by the deposited Al atoms, thus leaving some Al-free regions, which can be nitridated into SixNy in step (2) when the N source is introduced. T1 is calculated as follows:
First, the upper limit TAl of T1 is the time during which the Si substrate surface is entirely covered by the Al atoms. However, it is much easier to obtain the AlN growth rate v than to obtain the TAl, e.g., through a reflectivity oscillation curve in MOCVD or by a RHEED in-situ monitoring device in MBE. Detailed description follows about how to calculate the upper limit TAl of T1 by the aid of the AlN growth rate v.
By definition, TAl means that the Si surface is just entirely covered by the Al atoms. If there was sufficient supply of the N source in the reaction chamber during TAl, the Al source would react with the N source to generate AlN; and when the kinetic processes of this reaction like diffusion, decomposition, absorption, surface migration, desorption and so on were so idealized that during TAl all the Al atoms covering Si surface were completely converted into AlN, the time for this idealized status was the upper limit of TAl, i.e., time for growing a single layer of AlN, TAlN Moreover, if the tension stress is ignored and the lattice constant of AlN at growth direction c=0.50 nm, it is easy to obtain TAlN=c/v through the AlN growth rate. Where units of TAlN and v are s and μm/h respectively, TAlN=1.8/v. To sum up, the exposure time of Al source, T1, should meet: 0<T1<TAl<TAlN=1.8/v, i.e., 0<T1<1.8/v.
Further, to expand the time window of supplying Al source in sub-step (i), it is required to deposit the Al atoms at a slow rate. That means, if sufficient N source was provided at this time, the AlN growth rate through reaction of the Al source and the N source would be slowed down accordingly; thus from 0<T1<1.8/v, v should be small and the window of T1 could be enlarged to improve the usability of the present disclosure. Generally, in sub-step (i) of supplying the Al source, it is appropriate to make v meet 0<v<1, and the growth conditions which guarantee a small v value can realize slow-rate Al atom deposition. For instance, the time window of step (i) can be enlarged through low-flow Al source.
Another important aspect is to obtain an interface structure of “coexistence of Al atoms and SixNy” after closing the Al source is in sub-step (ii) which introduces in the N source for an appropriate period of time denoted by T2. T2 depends on different epitaxial methods and equipment. According to some embodiments where MOCVD is adopted, the T2 meets 0<T2<5/FNH3, wherein, FNH3 is the NH3 flow on each square centimeter of the substrate, and units of FNH3 and T2 are slm/cm2 and min respectively.
Then, in sub-step (iii) the Al source and the N source are provided simultaneously for a period of time T3 to grow AlN of a certain thickness, so as to prevent the potential Ga meltback etching on the Si substrate. In general, T3 should guarantee that the thickness of the AlN epitaxial layer, HAlN meets 1 nm≦HAlN≦500 nm.
The last step is to further grow the Group-III nitrides over the above structure. The Group-III nitrides may comprise a single-layer or multi-layer of following Group-III nitrides: AlN, GaN, InN, AlxGa1-xN, AlxIn1-xN, InxGa1-xN or (AlxGa1-x)1-yInyN, where, 0<x<1, 0<y<1.
In addition, except for the foresaid growth methods like MOCVD, MBE or HYPE, other growth methods are also applicable.
Refer to
The present disclosure will describe in detail taking MOCVD growth method as an example.
For the MOCVD growth method, the Al source and the N source are TMAl and NH3 respectively. The interface structure of “coexistence of Al atoms and SixNy” can be formed in situ mainly through the following three steps: step 1 introduces in the TMAl for an appropriate period of time T1, such that some regions of the Si surface are covered with the Al atoms and other regions are not as the exposure time to TMAl is short; step 2 closes TMAl and opens NH3 for an appropriate period of time T2, thus the Si surface not covered with Al atoms will be nitrided into SixNy, and those regions covered with Al atoms are free from nitridation, and meanwhile some Al atoms may be nitrided into AlN; in step 3, introducing in the TMAl and the NH3 simultaneously for T3 to grow AlN to a certain thickness, so as to prevent potential Ga meltback etching on the Si substrate.
In step 1, the TMAl introduction time T1 should meet 0<T1<1.8/v. To expand the time window of step 1 in the MOCVD, it is required to deposit the Al atoms at a slow rate, which is realized by controlling the growth conditions of step 1. In general, small v value and corresponding slow Al atom depositing can be realized through one or more such growth conditions as low-flow TMAl, high-pressure, or carrier gas with high H2 ratio. Specifically, TMAl flow FTMAl should meet: FTMAl≦20 μmol/min·cm2, wherein: FTMAl is the TMAl flow over each square centimeter of substrate per minute and the unit is μmol/min·cm2; high pressure P should meet: P>30 Torr; and carrier gas with high H2 ratio should meet: FH2/(FH2+FN2)≧0.3, wherein: FH2 and FN2 are flows of carrier gases H2 and N2 respectively.
In step 2, as to the appropriate time T2 to introduce in NH3 after closing the TMAl, generally for MOCVD, the structure of “coexsistence of Al and SixNy” can form well where T2 meets 0<T2<5/FNH3, wherein, FNH3 is the NH3 flow on each square centimeter of the substrate, and units of FNH3 and T2 are slm/cm2 and min, respectively.
In addition, as to the AlN epitaxial time T3 in step 3, in general, T3 should guarantee that the thickness of the AlN layer, HAlN, meets 1 nm≦HAlN≦500 nm.
Refer to
Refer to steps below for growing Group-III nitrides over the Si substrate by MOCVD: a) Si surface pre-processing; b) H2 high-temperature processing in chamber; c) epitaxy of the interface structure of “coexistence of Al atoms and SixNy”; and d) further epitaxy of the Group-III nitrides.
a) surface pre-processing of Si substrate outside the reaction chamber;
Pre-processing for the Si substrate surface is performed according to standard procedures of RCA clean. The RCA clean mainly comprises the three sub-steps below: i. removal of the organic contaminants on Si surface with mixed solution of NH4OH and H2O2; ii. removal of thin oxide layer with HF solution; and iii. removal of metal ionic contamination with mixed solution of HCl and H2O2. In addition, DI water cleaning is needed after each step.
b) High-temperature H2 baking in the MOCVD reaction chamber;
After RCA clean, place the Si substrate in the MOCVD reaction chamber, and raise the temperature to about 1,100° C. for further baking and cleaning of the Si substrate in H2 ambient.
c) Epitaxy of the interface structure of “coexistence of Al atoms and SixNy”;
This structure is formed mainly through the three steps below:
I. Introduction of TMAl for T1.
Under grown conditions of 1,100° C., 50 Torr and FH2 (FH2+FN2)=0.5, where the Al source TMAl flow per square centimeter of substrate is 5 μmol/min·cm2 and the N source NH3 is sufficient, the corresponding AlN growth rate by MOCVD is v=0.1 μm/h and correspondingly 0<T1<1.8/v, i.e., 0<T1<18 s. Therefore, under the above growth conditions, the TMAl can be turned on in step 1 for T1=8 s.
II. Introduction of NH3 for T2 after closing TMAl.
Where the N source NH3 flow per square centimeter of substrate is 0.2 slm/cm2, the time T2=5 min to make Si surface partially deposited with Al atoms.
III. Simultaneous opening of TMAl and NH3 for T3 for AlN epitaxy.
Under the growth conditions where AlN growth rate is about 0.5 μm/h, grow AlN for T3=24 min and obtain AlN about 200 μm.
d) Further epitaxy of the Group-III nitrides over the above structure. The Group-III nitrides comprises a single-layer or multi-layer structure of following Group-III nitrides: AlN, GaN, InN, AlxGa1-xN, AlxIn1-xN, InxGa1-xN or (AlxGa1-x)1-yInyN, where, 0<x<1, 0<y<1.
All references referred to in the present disclosure are incorporated by reference in their entirety. Although specific embodiments have been described above in detail, the description is merely for purposes of illustration. It should be appreciated, therefore, that many aspects described above are not intended as required or essential elements unless explicitly stated otherwise. Various modifications of, and equivalent acts corresponding to, the disclosed aspects of the exemplary embodiments, in addition to those described above, can be made by a person of ordinary skill in the art, having the benefit of the present disclosure, without departing from the spirit and scope of the disclosure defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent structures.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201310339115.9 | Aug 2013 | CN | national |
The present application is a continuation of, and claims priority to, PCT/CN2014/081768 filed on Jul. 7, 2014, which claims priority to Chinese Patent Application No. 201310339115.9 filed on Aug. 7, 2013. The disclosures of these applications are hereby incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CN2014/081768 | Jul 2014 | US |
| Child | 15016261 | US |
