Epitaxial wafer and semiconductor memory device using the same
An epitaxial wafer and a semiconductor memory device, the epitaxial wafer including a semiconductor substrate having a front surface and a rear surface opposite to each other; a strain relaxed buffer (SRB) layer on and entirely covering the front surface of the semiconductor substrate; and a multi-stack on and entirely covering a surface of the SRB layer, wherein the SRB layer includes a silicon germanium (SiGe) epitaxial layer including germanium (Ge) at a first concentration of about 2.5 at % to about 18 at %, and the multi-stack has a superlattice structure in which a plurality of silicon (Si) layers and a plurality of SiGe layers are alternately provided.
1. An epitaxial wafer, comprising:
a semiconductor substrate having a front surface and a rear surface opposite to each other;
a strain relaxed buffer (SRB) layer on and entirely covering the front surface of the semiconductor substrate; and
a multi-stack on and entirely covering a surface of the SRB layer,
wherein:
the SRB layer includes a silicon germanium (SiGe) epitaxial layer including germanium (Ge) at a first concentration of about 2.5 at % to about 18 at %, wherein the first concentration is an amount that causes a lattice constant of the SRB layer to be between a lattice constant of the semiconductor substrate and an overall lattice constant of the multi-stack,
the multi-stack has a superlattice structure in which a plurality of silicon (Si) layers and a plurality of SiGe layers are alternately provided, and
the overall lattice constant is an effective lattice constant of the multi-stack that depends at least on:
a thickness of a respective Si layer of the plurality of Si layers;
a thickness of a respective SiGe layer of the plurality of SiGe layers;
a lattice constant of the respective Si layer; and
a lattice constant of the respective SiGe layer.
2. The epitaxial wafer as claimed in claim 1 , wherein:
each of the plurality of SiGe layers included in the multi-stack includes Ge at a second concentration of about 10 at % to about 30 at %, and
the first concentration is less than the second concentration.
3. The epitaxial wafer as claimed in claim 2 , wherein a thickness of the SRB layer is at least 2 μm.
4. The epitaxial wafer as claimed in claim 3 , wherein a degree of relaxation to lattice strain of the SRB layer is about 95% or more.
5. The epitaxial wafer as claimed in claim 1 , wherein:
a number of Si layers included in the multi-stack is at least 80, and
one Si layer of the plurality of Si layers is at an uppermost end of the multi-stack.
6. The epitaxial wafer as claimed in claim 5 , wherein:
each Si layer of the plurality of Si layers included in the multi-stack includes an epitaxial growth layer, and
each SiGe layer of the plurality of SiGe layers included in the multi-stack includes an epitaxial growth layer.
7. The epitaxial wafer as claimed in claim 1 , wherein the plurality of Si layers and the plurality of SiGe layers included in the multi-stack are substantially defect-free.
8. The epitaxial wafer as claimed in claim 1 , wherein a lattice constant of an upper layer of the SRB layer is substantially the same as the overall lattice constant of the multi-stack based on the superlattice structure of the multi-stack.
9. The epitaxial wafer as claimed in claim 1 , wherein the semiconductor substrate includes a Si single-crystal substrate.
10. The epitaxial wafer as claimed in claim 9 , wherein the semiconductor substrate includes an oxygen (O) element and a group III element or a group V element as impurities.
11. An epitaxial wafer, comprising:
a semiconductor substrate having a front surface and a rear surface opposite to each other;
a strain relaxed buffer (SRB) layer on and entirely covering the front surface of the semiconductor substrate; and
a multi-stack on the whole surface of the SRB layer,
wherein:
the SRB layer includes a silicon germanium (SiGe) epitaxial layer including a lower layer and an upper layer,
the lower layer has a concentration gradient of germanium (Ge) therein,
the upper layer has a uniform concentration of Ge, the uniform concentration being a first concentration of about 2.5 at % to about 18 at % wherein the first concentration is an amount that causes a lattice constant of the SRB layer to be between a lattice constant of the semiconductor substrate and an overall lattice constant of the multi-stack,
the multi-stack has a superlattice structure in which a plurality of silicon (Si) layers and a plurality of SiGe layers are alternately provided, and
the overall lattice constant is based on the superlattice structure of the multi-stack and depends at least on a lattice constant of a respective Si layer of the plurality of Si layers and a lattice constant of a respective SiGe layer of the plurality of SiGe layers.
12. The epitaxial wafer as claimed in claim 11 , wherein:
a concentration of Ge in a portion of the lower layer of the SRB layer in contact with the front surface of the semiconductor substrate is about 0 at %, and
a concentration of Ge in a portion of the lower layer of the SRB layer in contact with the upper layer is about the same as the first concentration.
13. The epitaxial wafer as claimed in claim 11 , wherein:
a thickness of the lower layer of the SRB layer is less than a thickness of the upper layer of the SRB layer, and
a total thickness of the lower layer and the upper layer of the SRB layer is at least 2 μm.
14. The epitaxial wafer as claimed in claim 11 , wherein a lattice constant of the upper layer of the SRB layer is substantially the same as a lattice constant of the multi-stack.
15. The epitaxial wafer as claimed in claim 11 , wherein:
each Si layer of the plurality of Si layers and each SiGe layer of the plurality of SiGe layers included in the multi-stack includes an epitaxial growth layer,
the multi-stack includes at least 80 Si layers, and
one Si layer of the plurality of Si layers is at an uppermost end of the multi-stack.
16. A semiconductor memory device, comprising:
a semiconductor substrate having a front surface and a rear surface opposite to each other;
a strain relaxed buffer (SRB) layer on and entirely covering the front surface of the semiconductor substrate, the SRB layer being formed by epitaxially growing silicon germanium (SiGe);
a plurality of single-crystal silicon (Si) layers on the SRB layer, the plurality of single-crystal Si layers having the same lattice constant as the SRB layer and being arranged at equal intervals,
wherein the SRB layer includes germanium (Ge) at a first concentration of about 2.5 at % to about 18 at %, wherein the first concentration is an amount that causes a lattice constant of the SRB layer to be between a lattice constant of the semiconductor substrate and an overall lattice constant of a multi-stack that includes the plurality of single-crystal silicon (Si) layers, the overall lattice constant based on a superlattice structure of the multi-stack and depending at least on a lattice constant of each respective layer of the multi-stack;
a bit line on the front surface of the semiconductor substrate and extending through the plurality of single-crystal Si layers in a vertical direction;
a transistor body portion including a first source/drain region connected to the bit line, a single-crystal channel layer, and a second source/drain region arranged sequentially in a first horizontal direction;
a gate electrode layer extending in a second horizontal direction orthogonal to the first horizontal direction and covering an upper surface and a lower surface of the single-crystal channel layer with a gate dielectric layer therebetween; and
a cell capacitor on an opposite side to the bit line from the transistor body portion in the first horizontal direction and including a lower electrode layer connected to the second source/drain region, a capacitor dielectric layer, and an upper electrode layer.
17. The semiconductor memory device as claimed in claim 16 , wherein:
a thickness of the SRB layer is at least 2 μm, and
a degree of relaxation to lattice strain of the SRB layer is about 95% or more.
18. The semiconductor memory device as claimed in claim 16 , wherein:
the plurality of single-crystal Si layers includes at least 80 single-crystal Si layers, and
the single-crystal channel layer is formed over at least some of the plurality of single-crystal Si layers.
19. The semiconductor memory device as claimed in claim 18 , wherein:
the single-crystal channel layer is a channel region of a cell transistor, and
the plurality of single-crystal Si layers are epitaxially grown layers and substantially defect-free.